The Present Perfect for general statements

[Alexey86]

Are there any options for using the Present Perfect (especially with modals) in general statements, or only the Present Simple is correct? For example:

1. (a particular situation) I can't go for a walk because it's already 9 pm and I haven't finished my homework yet.
(a general statement) I can go for a walk only if I have finished/finish my homework before 9 pm.

2. (particular) He has known her for a long time, but still doesn't know her age.
(general) One may (can) know/have known someone for a long time and yet not know their age.

3. (particular) I don't trust him yet because I've known him only for a short time.
(general) I can trust somebody only if I have known/know them for a long time.

I would appreciate your help.

 

[emsr2d2]

For me, sentence #1 can use either tense, sentence #2 can use either tense (but "can" is better than "may") but sentence #3 only works with the present perfect.

 

[Alexey86]

Thank you. I didn't know that "can have done" was a possible construction at all, and especially in general statements that don't describe possible events in the past.
As for #3, what is the difference that makes only the Present Perfect suitable?

 

[abaka]

In (1) I prefer the simple present. The perfect is correct but may sound unnecessarily precise in ordinary speech.

In (2) the "have known" is probably better because "a long time" has already passed.

In (3) the present makes no sense because the (completed, ie. perfect) acquaintance "for a long time" is a necessary condition for your trust, and therefore requires the present perfect.

What is the difference between (1) and (3)? That's a hard question. Homework is either done or not done, and the period of time necessary to complete it is of no importance in (1). On the other hand, the completed length of the period of acquaintance is crucial in (3). I think perhaps that is the deciding factor in (3) as it is in (2).

 

[Alexey86]

Thank you! Does "can have done" sound fine to your ears? Do you hear or use such a construction in your everyday conversations?

In (2) the "have known" is probably better because "a long time" has already passed.

Do you think it's acceptable to consider this statement indefinite regarding time (timeless), since it's neither about a factual situation nor about an imaginary one like in conditional sentences?

 

[emsr2d2]

You keep asking about "can have done". Your sentence did not include that. In your sentence "One can have known someone ..." is perfectly acceptable. I'm not saying that "can have done" is not acceptable. It is - in the right context.

 

[Alexey86]

Sorry for the confusion. I'm using "can have done" as a general substitute for "can have known". I'm a bit confused, because I can't remember ever hearing such a construction.

 

[emsr2d2]

One can have done many things in life but there is always more to do.

 

[Alexey86]

One can have done many things in life but there is always more to do.

I googled it but couldn't find a match. I've found this thread on Quora, https://www.quora.com/What-is-the-difference-between-“can-have-done”-and-“could-have-done”. Most of the commentators think "can have done" is an impossible and illogical construction.

 

[GoesStation]

Most of the commentators think "can have done" is an impossible and illogical construction.

It's not at all natural in American English. I'm pretty sure I've seen it before now in the British variety.

 

[Alexey86]

It's not at all natural in American English. I'm pretty sure I've seen it before now in the British variety.

On the other hand, "can't have done" is used when the listener is so surprised or annoyed by what happened that he refuses to believe it. For example, He can't have done it. = I refuse to believe that he did/has done it.

​

 

[GoesStation]

On the other hand, "can't have done" is used when the listener is so surprised or annoyed by what happened that he refuses to believe it. For example, He can't have done it = I refuse to believe that he did/has done it.

​

That's not impossible in American English, but we're much more likely to say he couldn't have done it.

 

[Alexey86]

but we're much more likely to say he couldn't have done it.

Then, how to distinguish whether this statement conveys the meaning of someone's inability to perform a particular action or just expresses surprise?

 

[jutfrank]

I've found this thread on Quora, https://www.quora.com/What-is-the-difference-between-“can-have-done”-and-“could-have-done”. Most of the commentators think "can have done" is an impossible and illogical construction.

No, they don't. Only one says it's incorrect, and she's wrong.

It is both possible and logical. I think the reason you've never noticed it until now is that it's not very often that one needs to use it. Your sentence 2 is one such example.

 

[jutfrank]

Then, how to distinguish whether this statement conveys the meaning of someone's inability to perform a particular action or just expresses surprise?

The listener interprets based on context, and on the manner of intonation.

 

[Alexey86]

No, they don't. Only one says it's incorrect, and she's wrong.

Not only one. There are five people against "can have done". And there is a writer/editor among them.

 

[jutfrank]

Are we reading the same thing? Have another look. Which five do you think are 'against' it? Which part of what they say makes you think that?

 

[GoesStation]

Then, how to distinguish whether this statement conveys the meaning of someone's inability to perform a particular action or just expresses surprise?

If it's not clear from context, the listener will understand he couldn't have done it as "I don't believe that he did it." I'd use different words to express the other idea: He wasn't capable of doing it, for example.

 

[abaka]

Thank you! Does "can have done" sound fine to your ears? Do you hear or use such a construction in your everyday conversations?

It's literary language -- high style, very cultivated speech.

Do you think it's acceptable to consider this statement indefinite regarding time (timeless), since it's neither about a factual situation nor about an imaginary one like in conditional sentences?

I began explaining #1 as a hypothetical rather than actual situation, but in the end decided that the degree of fact or hypothesis is not really decisive in your three sentences. If anyone chooses to argue that it is, I should be very happy to consider the argument!

 

[Alexey86]

Are we reading the same thing?

I hope so If you scroll the page down you’ll see them. There are even 6 people against, not 5.


#1 Sarah Madden, Writer/editor working in the gig economy
Answered Nov 24, 2017

The difference between your two examples is that the first one is incorrect, and the second one is correct.
You cannot mix the present tense (“can”) with the perfect tense (“have done”). The word “could” is the subjunctive (mood ofpossibility), so it fits better in this example. Here’s an example:“I could have done all the assignments if I had found enough time last week.”

#2 Pamela B.Zohar, lives in Athens, GA (2018-present)
Answered Nov 25, 2017

There is some difference - but mainly, if I‘have done’ something, I would use ‘could’ instead of ‘can’.
Can implies that I am ABLE to do something - but if this ‘something’ occurred in the past (‘have done’) then I ought to know whether I actually did it or not, correct?
So ‘I COULD HAVE DONE’ something (but I didn’t) or ‘I DID SOMETHING’ (because I did do it).
‘Can have done’ is illogical. If I did it, I know I did, and if it didn’t, then the only thing I can say (present tense) is I COULD HAVE done it. Maybe.

#3 Robert Ball, studied at College of the Holy Cross (1973)
Answered Nov 23, 2017

can have done makes no sense. It mixes tenses in an unintelligible way.
You would say I could have done that, meaning that was a possible course of action in the past that you did not take.
If you want to talk about the present or future, you would not use have, you would simply say I can do that, which means that is a course of action you are capable of taking, but have not yet taken.

#4 Sean Stockemer, Learning German made me more aware of my native tongue.
Answered Nov 23, 2017

The most blatant difference is that ‘can have done’ is incorrect.
Somebody correct me if I’m wrong, and I’ll delete this answer, but I cannot think of an example sentence in which I could use that phrase.

#5 Frans Delt, Persian Lecturer of English Linguistics/Living in Holland
Answered Nov 23, 2017

The former is grammatically incorrect and the latter is grammatically correct.

#6 Doris Arnold, former teacher (1970-2008)
Answered Nov 25, 2017

Could needs to be correct because could is past tense and done is past tense.