+4
Hello,

I have two questions concerning making deductions with the modal verbs.

can't have/couldn't have
  1. She can't have done that. She was abroad.
(I mean it was physically impossible that she has done that)

  1. She couldn't have done that. She is not such a person.
Am I right if I think that 'can't have' means physical impossibility, whereas 'couldn't have' concerns more somebody's character, etc. ?

My second question concerns could/might/may have + past participle

I think all of them mean the same:
  1. She could/may/might have lost your telephone number. (=it's possible that she has lost your number)
'Might' means that the possibility was very little, whereas 'may' and 'could' express the same likelihood. Am I right?

Would be grateful for your help,

Best regards.
Comments  
Modals are used to express doubt, uncertainty, tentativeness, conditionality, etc., that is, when you use them, you are not making a strong, definitive statement. Therefore, a statement like, "She can't have done that. She was abroad.", does not mean absolute impossibility that she didn't do it. The use of a modal always gives some tentativeness and doubt to the statement. If you wanted to say absolutely that she didn't do it, then you would not use a modal. Instead, you'd say something like: "She did not do that. She was abroad."

Similarly, the statement, "She couldn't have done that. She is not such a person.", again leaves an element of doubt about whether or not she did it. It's the modal that does this. Do not use any kind of modal if you want to make a definitive statement: "She did not do that. She is not such a person."

The following have a similar meaning:

She could have lost your number.

She may have lost your number.

She might have lost your number.

You'd say something like this in the context of someone not calling as expected. The reason - other than her simply not liking you - is that there is the (rather remote) possibility she lost your phone number. The hierarchy in these three sentences is subtle.

"She may have lost your number." probably holds out the most possibility that she actually lost the number.

"She might have lost your number." probably holds out less of a possibility that she actually lost the number and doesn't simply not like you.

"She could have lost your number." is getting into sarcasm, almost: she probably just doesn't like you, but oh, there is still the (very remote) possibility that she actually lost the number.
Hi.

You have the right impression about your first question.

As for the second question, may and might act identically in your context, but could is a different word here.

Could express the possibility but may and might express probability. Something could have happened, but at the same time it may not have happened.

So when you are using could, you state that sth was possible but you are not talking about its likelihood, but may and might are concerened with the odds for or against the idea.

I hope I could be of any help
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Thank you.

As for the first question (can't have/couldn't have):

According to the rule:

She can't have heard you correctly. (She is almost deaf)

She couldn't have heard you correctly. (She wouldn't have answered in such an unkind way)

Are 'can' and 'could' sometimes used interchangeably? For example:

She couldn't have heard you correctly. (She is deaf)

As for the second question, I find it difficult to understand the difference between 'possibility' and 'probability'. If I understand well:

She could have lost your number. (As she is a scatty person, it has already happened to her, etc.)

She may have lost your number (It is possible that she she has lost the number)

Thank you very much Emotion: smile