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Ok, this site says:

"Couldn't have" is used to show someone's feeling that something in the past is impossible.

Examples:

You couldn't have seen Jerry yesterday. He's been out of town since last Tuesday!

(It's impossible for you to have seen Jerry yesterday because he wasn't here. He's been out of town since last Tuesday!)

So, "someone's feeling" is a guess right? Because if that actually happened, then you use "Couldn't do" right?

Let say we all know that Tom is a very lazy student

Suppose if Tom actually failed the test (he got the result in front of himself) & we saw his failed result, we will say: "Tom couldn't pass the test"

but suppose Tom took the test & he will get his result tomorrow, then we will say: "Tom can't pass the test"

however, suppose Tom took the test & he got the result in front of himself, we know he already got his result but we don't know if he passed the test or not, we will say: "Tom couldn't have passed the test"

However, thing will completely opposite if we say:

"Tom couldn't have passed the test without cheating", that means Tom actually passed the test (he got the passed result)

Am I thinking right?

Modal verbs are the hardest English stuff I ve ever learned.

  • "Tom couldn't have passed the test without cheating" can mean he took the test or he didn't, and if he took it, his success was due to cheating. But the sentence could also be used to suggest that he had not taken the test, but that if he had the only way he could have passed would have been to cheat. – Robusto Feb 4 '17 at 14:36
  • This could be an interesting read. pearsonlongman.com/ae/azar/grammar_ex/message_board/archive/… – Ahmbro Dude Feb 4 '17 at 18:07
  • I think you’re thinking right. – J.R. Feb 23 '17 at 9:24
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The when someone says couldn't have, they're stating that it was impossible, that it was not able to happen. They might be mistaken.

Tom couldn't have passed the test.

This is different from

Tom didn't pass the test.

and

Tom couldn't pass the test.

The first is saying that there was no possibility that Tom could have passed the test. Maybe Tom took the test and failed. Maybe Tom didn't even take the test. Tom was not able to pass the test.

In the second, Tom did take the test and the speaker knows the result. Maybe it was possible or impossible, maybe he was able or unable, the sentence doesn't say, it just says that he failed.

The third sentence is saying that Tom tried, but was ultimately unsuccessful. He was unable to pass. If we know the result and say Tom couldn't pass the test, that's saying that he lacked the ability.

Tom couldn't have passed the test without cheating.

If Tom did actually pass the test, it could only have been by cheating. Maybe he didn't even take the test, because he knew he would fail and was not prepared to cheat. Maybe he passed, but wasn't able to without cheating.

couldn't have is only a statement about possibility or ability. It does not have to be a guess. It could be backed up by facts, or it could just be a feeling.

You couldn't have gone to the USSR last year - it broke up in 1991. (fact)

You couldn't have met the Queen last year - you're not important enough! (opinion)

could have can be used to express a possibility, rather than an impossibility with couldn't have.

Tom didn't pass the test. He could have passed if he tried harder.

The speaker believes that Tom was able to pass the test but didn't.

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"Couldn't have" is used to show someone's feeling that something in the past is impossible.

I disagree.

It's true that when speaking in the future tense "can" or "could" are just possibilities. For example "you could go to the fair tomorrow" means it is an option open to you. But when you say "cannot" or "could not" (contracted to couldn't) you are denying it, so it becomes affirmative. You are saying that there is definitely no possibility.

So likewise in your example which is in the past tense:

You couldn't have seen Jerry yesterday.
or
You could have seen Jerry yesterday.

Although you are speaking about something that didn't happen either way - your friend did not see Jerry - there is a difference because could have means it was possible, even though they didn't take it; but couldn't have means that there was definitely no possibility of seeing Jerry.

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