Can't have p.p. vs. couldn't have p.p.

Question

For me, a little bit of blurriness persists between the two.
Can anybody draw a clean-cut line between them? Pleeeese help.

(1)He can't have seen the movie.
(2)He couldn't have seen the movie.

(3)He can't have seen the movie last month.
(4)He couldn't have seen the movie last month.

(5)He can't have arrived by now.
(6)He couldn't have arrived by now.

Which of the pair would you native speakers use?

CalifJim · Accepted Answer

I think I say "couldn't" in this context more often than "can't". The two are virtually identical; however, "can't" is more objective and concerned with the laws of logic, I would say.

"He can't have seen the movie" sounds like a detective's remark, a logical deduction from other evidence in the case, and so does "He must not have seen the movie", another equivalent!

.

"He couldn't have seen the movie" sounds more everyday, and slightly more tentative (or conditional?) in view of the fact that "couldn't have" can mean "wouldn't have been able to". "would be" is always more tentative than "is".

That said, you will hear all of these in various contexts. They all say that it is not possible that he has seen the movie, or that the only logical conclusion from the evidence is that he has not seen the movie, which amounts to practically the same thing.

Your other pairs of sentences can certainly be analyzed in the same way.

Komountain · Answer

Than you.

Is it okay if I understand the phrase "last month" in (3) and (4) has nothing to do with the choice between "can't" and "couldn't"?

I cite one question from an ESL website. No situation was provided. Just a truncated sentence was.

"I ___ have read it last year."

a. mayn't
b. couldn't
c. can't

They say the answer is b. Can I just write this question off as weird?
If b is a correct answer, what's the logic behind this?
I don't have any intention to put you on the spot. I stand on your side.

CalifJim · Answer

No, it's not just weird. I imagine that most speakers feel "could" is more appropriate than "can" in sentences which refer to the past explicitly, as in "last year". Personally, I think c. is a possible answer as well, but as I said, it gives the impression of emphasizing the logic of the situation. "I can't have read it last year. It wasn't even written until last month!"

For test-taking purposes, "couldn't" certainly could be considered a "better" answer without being the "only" answer.

Please understand that others could have a different approach to your question!

Anonymous · Answer

So clear and billiant. Thx!!

Steve Blake · Answer

I would like to share an idea or even try to answer this question.

"I ___ have read it last year."

a. mayn't
b. couldn't
c. can't

IF I were to choose answer, I would go for letter B which says COULD NOT.

However, let me expound the given question a little bit. It would have been very clear if you had written it in a conditional sentence.

For instance:

IF the letter I received last year was written in
Japanese, I "couldn't have read (red) it.

This would mean that you "would not have been able to read (reed) the letter (last year).

You may also say: If I had a chance to read (reed) the said letter which was written in Japanese, I wouldn't have read it. Who would want to read it?

I hope that helps. Cheers

Anonymous · Answer

komountain For me, a little bit of blurriness persists between the two.
Can anybody draw a clean-cut line between them? Pleeeese help.
(1)He can't have seen the movie.
(2)He couldn't have seen the movie.
(3)He can't have seen the movie last month.
(4)He couldn't have seen the movie last month.
(5)He can't have arrived by now.
(6)He couldn't have arrived by now.
Which of the pair would you native speakers use?

Can't have p.p. vs. couldn't have p.p.

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