5
  1. That can't be Obama at the door, it's too early.

  2. That couldn't be Obama at the door, it's too early.

What is the difference between them? Are they almost the same?

  • I've got the (a?) feeling that this use of "couldn't" in sentence 2 is not exactly correct. – CowperKettle Nov 30 '14 at 9:00
  • @CopperKettle "isn't" > "can't be" > "couldn't be" -- that would be an ordering of strength, roughly speaking, . . . :) – F.E. Dec 1 '14 at 1:27
2

These sentences are grammatically correct, with a little difference in meaning.

1- That can't be Obama at the door; it's too early.

When the speaker says so, he means that he is sure that the person at the door is not Obama as it's too early.

2- That couldn't be Obama at the door; it's too early.

Here the speaker means it's not or it might not be possible that the person at the door is Obama as it's too early.

(Pls refer to Oxford LD to look up the use of can (7) and could (4).

  • Correct. Can't is short for can not. "Can" is the indicative mood of the verb, and states actual fact. Couldn't is short for could not. "Could" is the subjunctive mood of the verb, and states conjecture, possibility or belief. – CXJ Dec 3 '14 at 15:52
1

They are similar both grammatically and lexically.

That can't be Tom - he's in Japan.
That couldn't be Tom - he's in Japan.

(These are both used to mean that the speaker doesn't believe that.)

That can't have been Tom - he was in Japan.
That couldn't have been Tom - he was in Japan.

(These are used to mean that the speaker didn't believe that.)

Similarly, please take a look at the examples below:

Tom can't be moving the furniture upstairs - he's at work right now.
Tom couldn't be moving the furniture upstairs - he's at work right now.

Tom bumped into me. He can't/couldn't have been looking where he was going.

Could do means the same thing as may/might. They all refer to uncertain possibility at present or in the future. However, could in this sense can only be used in the positives and questions, not in the negatives.

It could/may/might rain tonight.

It may/might not rain tonight.

(Could not is impossible in this case.)

(Someone is knocking at the door and you're answering the door.)

Who could that be?

(May/might are impossible in this case.)

"Have some more cake."
"No, I couldn't."

1
  • That can't be Tom, it's too early.
  • That couldn't be Tom, it's too early.

In the above two examples, both statements are set in the present tense, but the first one is a definite statement of fact, while the second statement is based on the implied impossibility of something, in other words, conjecture.


I think "couldn't" can refer to either future hypothetical scenarios, or past hypothetical scenarios, suggesting the likely impossibility of something.

  • For example:
  • Q: If they offered you a huge sum of money, would you be willing to do it?

  • A: I couldn't do it; not even if they offered me all the money in the world.

In the above instance, "I couldn't do it" refers to an implied action that the person claims would be hypothetically impossible for them to carry out in the future. This is an example of a type 2 future conditional, involving the use of a modal.

"I couldn't have done it" on the other hand, refers quite clearly to the past.

Sure thing! Reference: https://www.ef.com/wwen/english-resources/english-grammar/type-2-conditional/

  • Thanks for your contribution. How about adding some reference? – WXJ96163 Apr 18 '20 at 8:46
-1

The word "can't" refers to a definite impossibility in the present tense. However, the word "couldn't" refers to a impossibility in the past. In this case, it is correct to say "That can't be Obama at the door, it's too early." It would also be correct to say "That couldn't have been Obama at the door, it was too early" since we would be referring to an event in the past.

More reference here: https://learnenglish.britishcouncil.org/en/english-grammar/verbs/modal-verbs/can-or-could

  • Could is not always for the past. On the same page: We use 'could' to show that something is possible in the future, but not certain. – Maulik V Oct 31 '14 at 5:24
  • "Could" could be used for the future, but not "couldn't". Nowhere in my answer do I talk about "could". – Krishna Ravi Oct 31 '14 at 18:14

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