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I've found some questions here about the can+have+V3 possibility when speaking about possible situations in the past. But there was nothing said about future perfect usage, and I wonder if we can use it to speak about some actions that can become accomplished in the future.

I know that we can use will or may/might in this case, but I feel that the meanings may be slightly different:

  1. I will have done it by tomorrow. - It sounds like a promise to me: I'm quite sure that I will accomplish it today.
  2. I may/might have done it by tomorrow - a possibility: I probably will accomplish it today, but I'm not sure at all
  3. *I can have done it by tomorrow - an ability: I have an opportunity to accomplish it today (but I may not use it)

Is the third sentence possible, and does it all make sense?

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  • Seems to me that "can have done" is impossible (although "cannot have done" works - see the linked question - and the interrogative "can I have done ..." seems to work too). Instead of your third sentence, we would simply say "I can do it by tomorrow". Interestingly, it would also be possible to say "I can have it done by tomorrow", but obviously that's a slightly different structure (more akin to "I can have it ready by tomorrow").
    – rjpond
    Commented Oct 27, 2017 at 21:36
  • Thanks. 'I can do it by tomorrow' will work. I was probably confused with "by tomorrow" and thought that it requires Perfect tense. Doesn't 'I can have done it by tomorrow' mean that someone will probably do it for me instead of that I will do it myself?
    – MartaPrelle
    Commented Oct 27, 2017 at 22:12
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    That's a good point: if you say "I can have it done by tomorrow" it leaves open the possibility that someone else might be doing the work - I wouldn't go so far as to say "probably". In some contexts it might be probable, in others improbable.
    – rjpond
    Commented Oct 27, 2017 at 22:18
  • She said, "I can have done it by tomorrow", rjpond; not "I can have it done by tomorrow." They aren't equal statements. I've never heard anyone say MartaPrelle's example; I've heard it with shall or will or even "am going to": "By this time tomorrow, I shall have done what you have asked of me" or "I shall have it done", but "have it done" is slightly different from "have done it". It's difficult. MartaPrelle, you'll just have to learn the subtle differences because it's hard for us native speakers to explain some of the highly technical points like this one.
    – Nick
    Commented Oct 31, 2017 at 2:35
  • "I shall have done it by tomorrow" and "I shall have it done by tomorrow" mean two separate things: the former is a prediction that between now and tomorrow, the project is going to "at some point" be finished, but it may be finished today or tomorrow; it's more of a general statement or general prediction. The latter means that I am going to cause it to happen by tomorrow; it's not so much a prediction as it is my stating that I'm going to cause it to be finished by tomorrow. It is more forceful. I'd be more assured of its being completed if you expressed the latter one to me.
    – Nick
    Commented Oct 31, 2017 at 2:46

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Here is definition 2c for "can" on freedictionary.com:

Used to indicate probability or possibility under the specified circumstances: "They can hardly have intended to do that."

I am a native speaker and I have asked myself this same question a few times when I have heard it; in fact, I just heard it a few days ago on an old television show from 1968 and every time I hear it on television, which is very rare, I flinch. The character said:

"He can't have gone far."

I personally disagree with the construction and would never say it since "could" is the past tense of "can" and this statement is talking about the past, i.e

"He left ten minutes ago, so he couldn't have gone far."

I suppose the question is whether it is grammatical. My answer is that I don't know; I've heard it before, but it doesn't make sense grammatically and it's rare to hear it or see it in writing. Would I use it? Never. I refuse to use something that is awkward like that when "could" works even better. It's your choice though. It's more heard when it's in the negative and "They can hardly have intended to do that" has a negative meaning.

I hope somebody else can answer this question better than I can. I'd appreciate a definitive answer.

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  • Well, my grammar book says that the difference between 'Can he really have said it?' and 'Could he really have said it?' is that the first one sounds more indignantly. It could be incorrect or outdated, so I'd prefer to know how native speakers feel about it. But my question was about the future perfect. From the previous comment, I assumed that native speakers would rather say 'I can finish it by tomorrow' while 'I can have finished it by tomorrow' sounds odd, although it can be grammatical.
    – MartaPrelle
    Commented Oct 30, 2017 at 21:37
  • Yes, you're right.
    – Nick
    Commented Oct 30, 2017 at 23:42
  • The future perfect is usually used with "shall" and "will" and means something different from the future simple. "I shall be there on Friday" means that my arrival is going to happen on Friday; however, "I shall have been there (by) Friday" means that my arrival is going to happen sometime between now and Friday.
    – Nick
    Commented Oct 30, 2017 at 23:45
  • In English, we very rarely talk in the future perfect; often the future simple can be used to convey the same meaning as the future perfect, especially if we should switch out a word or two here or there. I can go a whole month and not say the future perfect in a sentence. If I do say it in that month, it's a handful of times. It is that uncommon. We use conditionals using should, could, would, might, or even may, plus "have" and then the past participle, but the future perfect with shall or will plus "have" and a past participle is very seldom said.
    – Nick
    Commented Oct 31, 2017 at 0:58
  • Look at the two sentences herein: "The incident will have occurred five years ago in February" can just be said as "The incident will be five-years-old in February." "I shall have worked for this company for five years on Saturday" can just be said as "On Saturday, it's going to be five years that I have worked at this company" or "On Saturday, I'll be entering my sixth year as an employee at this company." Personally, I try to avoid the future perfect in speech and I find that other native speakers do as well. I wrote the future perfect in a paper last week and it shocked me to see it.
    – Nick
    Commented Oct 31, 2017 at 2:26

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