"Could have" = "would possibly have" here?

Question

The following parts are taken from Michael Swan's, Practical English Usage (hereafter PEU):

259.2 could have ... and might have ...

We can use could have + past participle to mean 'would have been able to .. .', and might have + past participle to mean 'would perhaps have ... ' or 'would possibly have .. .'.

Example A: If he'd run a bit faster, he could have won.

Example B: If I hadn't been so tired, I might have realised what was happening.

But in 122.7 could have ...

We use this structure to talk about unrealised past ability or opportunities - to say that somebody was able to do something, but did not try to do it; or that something was possible, but did not happen.

Example C: I could have married anybody I wanted to.

Example D: I was so angry I could have killed her!

Example E: Why did you jump out of the window? You could have hurt yourself.

Example F: I could have won the race if I hadn't fallen.

Sometimes I feel it a little difficult to decide, for a given context, which of the following meanings of "could have" is being used:

• 'would perhaps have ...'
• 'would have been able to...'
• 'would have been allowed to... '
• 'would have the opportunity to ...'

I'm also not sure if could have can also mean "would possibly have". No grammar books explictly confirm this. Maybe the bold section in 122.7 suggests this (but I am not quite sure). So can I make the following substitutions (SUB) -- taken from the PEU text referenced above -- or would they change the original meaning or create an ambiguity?

• EXAMPLE B ORIG: If I hadn't been so tired, I might have realised what was happening.
• EXAMPLE B SUB: If I hadn't been so tired, I could have realized what was happening.
• EXAMPLE E ORIG: Why did you jump out of the window? You could have hurt yourself.
• EXAMPLE E SUB1: Why did you jump out of the window? You would possibly have hurt yourself.
• EXAMPLE E SUB2: Why did you jump out of the window? You might have hurt yourself.
• EXAMPLE F ORIG: I could have won the race if I hadn't fallen.
• EXAMPLE F SUB: I would possibly have won the race if I hadn't fallen.

Answer 1

Could have is saying that something was definitely possible in given circumstances.

Could have indicates that the option was available: that the possibility existed, not limited by circumstances, situation or personal ability. It tends to be used when you are talking about an entire hypothetical scenario.

When I was training for the marathon, I could easily have beaten you in a race!

The focus is on the capability: in this case the speaker knows that they definitely could have won the hypothetical race if such a race had occurred.

Note that saying they are definitely capable of doing so doesn't necessarily mean that it would definitely have happened, just that it had a probability of more than zero. A negative example might help to illustrate this:

Even if I had trained for a year, I could not have beaten Jesse Owens in a race.

It also doesn't give any indication of how likely the speaker thinks the possibility was, just that they believe it was definitely a possible outcome.

Would have defines a definite outcome given different circumstances.

Would have indicates that something intervened to stop the predicted outcome from arising. It tends to be used when you are talking about a change to the real sequence of events: if something had happened differently.

I would have beaten you in that race if I hadn't tripped up!

This is expressing certainty: if the situation was different (in this case, not tripping up), the predicted outcome (beating you) would definitely have occurred.

Might have defines a possible outcome given different circumstances.

Might have is the same as would have, but the speaker is less certain of the outcome.

I might have won that race if I had done more training.

I didn't win the race, I think I might have won if I'd done more training, but I also think that I still might not have won even if I had done more training.

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Your specific examples

As you might have guessed, many of these sentences are correct but with subtle differences in nuance.

Example B

• If I hadn't been so tired, I might have realised what was happening.
• If I hadn't been so tired, I could have realised what was happening.

Because this is talking about a change from the actual situation, not an entire hypothetical scenario, "might" sounds more natural here than "could". That doesn't mean that "could" is wrong, just that it would be more common to say "might".

If you were sure that you would have realised what was happening if you'd been less tired, you could say "would" instead of "might".

Example E

• Why did you jump out of the window? You could have hurt yourself.

This is indicating that hurting yourself was definitely an option (and hence it was stupid to jump out of the window). For this reason, it expresses the sentiment most strongly out of all your "window" sentences. It's implied that you were lucky not to hurt yourself - in other words, luck was all that prevented you from hurting yourself.

• Why did you jump out of the window? You might have hurt yourself.

"I suppose you might have hurt yourself, but I'm not a window-jumping expert, so I don't know..." There's less certainty here, so it's less forceful. However, it's still entirely acceptable.

• Why did you jump out of the window? You would possibly have hurt yourself.

"Would possibly have" has the same meaning as "might have". To me, it sounds slightly less natural here, but I think that's because the question itself sounds a little accusatory, so adding the "possibly" in is a little awkward.

Example D

You didn't suggest a substitution for D but I'm including it to show the difference from E.

• I was so angry I could have killed her!
• I was so angry I might have killed her!

In this instance, it's actually the "might" example that is stronger. That's because the "could have" suggests that the speaker was angry enough to kill her, but chose not to. Because it's "could", we know it's a hypothetical scenario and that there was no chance of him actually killing her: just that his anger gave him the capability of doing so.

However, "might have" admits the possibility that this outcome might actually have occurred. It sounds like it's something external to the speaker that prevented it, like someone walking in, or maybe he went so far as to beat or shake her and it was only luck she didn't die.

Needless to say, "would have" is therefore the strongest of the three, indicating that she definitely would have died if something had been different (like they didn't get interrupted).

Example F

• I could have won the race if I hadn't fallen.
• I would possibly have won the race if I hadn't fallen.

These both sound equally natural, but the nuance is different. In the first one, the speaker is certain of their ability to win races, but acknowledges that that doesn't mean they'll win every race they ever run. Falling stopped them being able to win the race - an ability they had before they fell.

In the second sentence, on the other hand, the speaker is still acknowledging that the outcome of the race (even without falling) was uncertain, but they are doing it in a way that includes the possibility they might not have won the race because they simply weren't good enough. It's a broader statement: that they might have gone on to win if they hadn't fallen, but they don't know if they would have gone on to win.

Some other things you mentioned in comments

I think your would possibly/perhaps have example is the same as might have, and would have been able to is the same as could have, with would have been allowed to being a subset of that where permission is the limiting factor of the "could".

Answer 2

This is an omphaloskeptical answer from a native American English speaker (North East). This is about the nuance I hear and observe, both locally in person (Boston, Massachusetts) but also in literature and among my fellow native English speakers on the Internet (including from Commonwealth countries).

Yes and no.

All of your proposed substitutions are grammatically valid and result in sensible sentences with very similar meanings to their originals.

They have subtly different emphases and nuance. The really big issue here is that could have used this way has a very strong negative valence, and is an (the?) idiomatic way of expressing blame or reproach. This is because(?) could have has the connotation of being about the capacities or responsibilities of the subject of the sentence, while would possibly have and might have are about the situation being described. When one says "I could have" one is not making a valence-neutral statement about how else a situation might have gone or would possibly have gone, one is discussing the role of "I" in how the situation did or did not go; "could have" attributes the situation to.... Well, read on.

Why did you jump out of the window? You could have hurt yourself. (Example E)

I could have won the race if I hadn't fallen. (Example F)

Therefore, I would think could have ... could also be reasonably replaced by would possibly have ... in Example E and in Example F. Am I right about this?

Yes, but in doing so, you would be shifting the statement from being about the capacities and responsibilities of the subject of the sentence to being about the circumstances the subject was in.

You could have hurt yourself.

...is a reproach, and is confronting someone with their negligence, while...

You would possibly have hurt yourself.

... is simply informatory. When your mother is yelling about your jumping off a bridge, she'll say "You could have hurt yourself!" When your orthopedist is self-congratulating about ordering that MRI and catching the wear on the cartilage in your knee such that it will be possible to repair with more minor surgery before you do damage to the bone, and you ask "Doc, what would have happened if we hadn't caught it in time?", she's more likely to say, "If we hadn't detected that in time and you had kept walking on it, you would possibly have hurt yourself."

Similarly...

I could have won the race if I hadn't fallen.

... is a self-reproach (or, occasionally, a reproach of another -- notice how in this famous example the speaker is describing his own capacities but ascribing the responsibility for the situation to another) while...

I might possibly have won the race if I hadn't fallen.

...is simply an observation of an alternate reality. In fact, it's so detached, I'm having trouble thinking of a situation where it would be used (since people seem to care a lot about competitive situations, and rarely are emotionally neutral about them!)

If I hadn't been so tired, I might have realised what was happening. (Example B)

[I]s it acceptable to substitute could have realized for might have realized in Example B[?]

Yes, however it makes it more of a reproach. The original (might have realized) communicates regret and possibly some self-blame, but could have realized makes that statement one of blame about whomever or whatever the speakers holds is responsible for causing them to be so tired. ("If [you|the refrigerator|my homework for that crummy stats class|my MLP:FiM habit] hadn't kept me up all night, I wouldn't be so tired.")

You could have hurt yourself. (Example E)

If so, is it acceptable to substitute [...] might have hurt for could have hurt in Example E without changing their original meaning?

Yes, it makes it less of a reproach. In fact, as part of my job I have to let people know when they're doing unwise things, and (I didn't realize this until I saw this question of yours, so I thank you for bringing this to my consciousness) I would deliberately use (and may have in fact used) might have hurt over could have hurt precisely to convey the same denoted information without so much reproach. Now, might have hurt still has some sting to it, so it's not something I'm ever likely to say unless I really need to confront someone pretty directly; at least it has some possibility of being moderated by tone of voice (if said in a very detached, low-key way), while could have hurt will always(?) communicate blame. "You could have hurt" is something I would approximately never say on the job.*

* The one time I clearly remember using "could have [hurt]" on the job, I was bringing to someone's attention the fact that their failure was potentially fatal to another person. And the verb was "killed." And I was totally willing for the person I was addressing to feel ashamed of themselves and to feel shamed by me.

Answer 3

Short answer

• "Could have" is the same as "Might have been possible"
• "Would have" is the same as "Might have chosen to"

Long answer

The difference between could and would is a difference of possibility and intention.

Given the sentence:

• I could have done my homework.

is saying that it was in my capability to do my homework.

Comparing that to:

• I would have done my homework.

is saying that I had the intention of doing my homework, but didn't. (usually would is followed with an explanation why a person chose not to)

Would possibly is not incorrect, butshould be avoided. Since "would have" is saying "I had the intention to" didn't, would possibliy is saying "I possibly had the intention to ".

Simply put, these three sentences are the same:

• I would possibly have walked the dog, but then it rained.
• I was unsure of my intention to walk the dog, but then it rained.
• I might have chose to walk the dog, I might have not chose to not walk the dog, but then it rained.

Which brings me to might have, which can be ether could or would. Might is simply saying the outcome is unknown.

Which means:

• "Could have" is the same as "Might have been possible to"
• "Would have" is the same as "Might have chose to"

Source: Im a native US English speaker. I have a bachelors degree, and have taken many English courses.

Caveat

All of the above are on technical grounds. In informal (and often formal) speaking "could have", "would have", and "might have" are all used interchangeably. In my personal experience most people can determine what you mean between the three using context.

Response

Would is directly a reference of will or intent. Strictly speaking some of the phrases you listed are grammatically incorrect. In everyday speech any given person wont know the difference, but as per the definition:

"would have been able to"

is grammatically incorrect because ability is a could, not a would.

"would have been allowed to" and "would have the opportunity to"

is also referring to ability not to intention or will.

The only one of those that is truly grammatically correct is:

"would perhaps have"

But similar to my previous statements, it's somewhat of a rare case. But it is still not synonymous with could have.

Answer 4

In British English, 'might possibly have' , 'would possibly have' and 'could possibly have' are tautological, because might/would/could already carry the uncertainty that possibly does, so it is an unnecessary word.

The construction with 'possibly' can be used as a morphological choice in speech to make a statement less forceful and so potentially less demanding, insulting or similar. It can convey a sense of the speaker feeling lower in stature than the person they are making the request of, hence the deliberately weaker request. In literature it would be used similarly but further aids in conveying the sense of a mild or weak character. e.g.

"Might I possibly play with you?" Asked the clumsy, bookish child of his classmates.

vs

"Might I play?"

The use of the past tense "might" still conveys a level of reserve (through formality) and uncertainty, so, even more confidently:

"May I play?"

There are also various British English idioms like "I couldn't possibly have another one", where the possibly is added for emphasis, but it is in emphasis of the negative.

As Identified by @Codeswitcher, might and could have different nuances, the former referring to the likelihood of something happening or a request for permission, the latter referring to the capacity of the agent/s in question being able to cause something to happen. Continuing with the example above:

May I play with you?

is a request for permission to play, whereas:

Can I play with you?

is a question asking whether in the questioned party's view the questioner has the ability/capacity to play and can be paraphrased to

Am I able to play with you?

Answer 5

To give you a simple answer:

• "Could" is the past tense version of the auxiliary connector "can" (which means "to be able to").

• "Would" is the past tense version of the auxiliary connector "will" (which gives the meaning that the action is guaranteed in the future). "Would" is the guaranteed future of the past, and as such, is usually used in expressing hypothetical past situations.

• "Might" is simply stating that the action is possible in the future, or that the hypothetical past event has the chance of having occurred. Shares similar implications with the word "maybe".

"Could have", therefore, gives the meaning that whatever was doing the action was really able to do that action.

"Would possibly have" gives the meaning that whatever was doing the action had only a possibility to guarantee that action in the future.

In short, are they equal? No.

Answer 6

Broadly yes--if you interchange them, the meaning would remain intact and the reader will understand what do you mean. We all agree that we can use could have + past participle to mean 'would have been able to .. .', and might have + past participle to mean 'would perhaps have ... ' or 'would possibly have .. .'

BUT...if you look it microscopically, the nuance of 'would have' and 'could have' is interesting. And here, I'm leaving the past participle thing aside. I may call it very minute difference of capability and possibility.

Let's take the first example:

If he'd run a bit faster, he could have won.

The use of both would and could is possible here and the sentence is clear to understand. However, putting could here shows the result of 'winning' through endurance or efforts. On the other hand, replacing it with ...might have won. would emphasize on possibility and not capability.

Now the second example:

If I hadn't been so tired, I might have realized what was happening.

Replacing might have with could have, again, would convey the message but if we see it the way I just narrated, there's a subtle difference. When you are not tired, you are in normal condition and you perceive everything easily and this comes naturally. Again, here, emphasizing on efforts or endurance of understanding something is not required. It is for this reason, might have is better than could have here. Change the second clause with the result that requires you being untired in the context of endurance, you'll find that could have works better -

If I hadn't been so tired, I could have done something with this flat tire right now.

Similarly, I could have married anybody I wanted to (shows your capability of being so confident); I was so angry I could have killed her (shows your anger and due to that your capability to kill); I could have won the race if I hadn't fallen (talks about your endurance and capability to win the race) --all talk more about your initiation, efforts or endurance in the context.

Let me think of conversion between two classmates as an example. They are meeting after a long vacation:

Jack: Hey Mike. How you doing?
Mike: I am okay. What about you?
Jack: I'm fine but upset. Lawrence won the lucky draw in an annual function of our college. I wished for you!
Mike: Yeah! I might have won. What all it required was luck!
Jack: Exactly. You missed it buddy. And yes, why did not you participate in 100 meter as well? You have good endurance and you are a good runner.
Mike: Again, my bad. You are right, I could have won it as well.