[Grammar] He couldn't /can't have done smth (modal verbs)

[tyrp]

Hello,
Modal verbs often cause trouble, and now I'm in a fix. Will there be any difference between the following sentences?
1. He can't have told you anything.
2. He couldn't have told you anything.
As I see it, the 1st sentence sounds more definite and categorical, more emotional than the second one, as 'can' generally is less doubtful than 'could'. Am I right? Will the 2nd sentence really sound less pressing and more polite or not?
And one more thing into the bargain. What form of the verb should be used in the case below?
He might have done his homework and not (a) left it for me / (b) leave it for me.
I guess the 1st variant (a) is more 'symmetrical' if you can say so about sentences, as the verbs are homogenuous: might have + past participle and not + past participle (an elliptical construction meaning: might have done smth and shouldn't have done smth else). But is the second variant definitely wrong? Is the construction 'natural' altogether?
Thanks in advance!!!

 

[Tarheel]

Interesting question, but I think you mean to say:

Is there any difference between the following sentences?
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Since no context is given I can only speculate about what the intent might be, but I can think of a possible usage.

Bob: Sam told me all about what happened.
Bert: He couldn't have told you anything. He's in a coma.
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As for the other one, perhaps:

He should have done his homework and not left if for me.
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(I don't know how you would use "leave" there.)

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[tyrp]

Thank you, dear Tarheel, but the first part of the question remains open.

1. He can't have told you anything.
2. He couldn't have told you anything.
As I see it, the 1st sentence sounds more definite and categorical, more emotional than the second one, as 'can' generally is less doubtful than 'could'. Am I right? Will the 2nd sentence really sound less pressing and more polite or not?

Is the suggested deduction correct? What's the difference in the use of the following sentences? Can we use BOTH of them in your sentence with coma?
E.g. "Bob: Sam told me all about what happened.
Bert: He couldn't / can't have told you anything. He's in a coma."
I'm sorry for splitting hairs, but the problem has been nagging at me for quite a while already. Thanks for your help!

 

[Tarheel]

"He can't have told you anything" might be possible. Longman thinks it is. See: https://www.usingenglish.com/forum/threads/62010-don-t-already-know

More on modals:
http://ifa.amu.edu.pl/~krynicki/teaching/pnjagr/res/12_modals.htm