About future probability:

It couldn't rain tomorrow.

It can't rain tomorrow.

It couldn't possibly rain tomorrow.

It can't possibly rain tomorrow.

About present probability:

It couldn't be true.

It can't be true.

May I ask what is the difference among those sentences? Are the meanings of the ones using "could" are less definite than the ones using "can"?

Please give me your opinion and I'll appreciate it.

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Comments  (Page 2) 
In this sense,

"I can take 100 calls a day" says that it is within your capacity to take 100 calls in a day. This one is more declarative.

On the other hand, "I could take 100 calls a day" says that you can probably do take 100 calls a day. This one is more of an answer to a request. ^^
If Could refer the 'possibility', what about the word 'may'. Could you please explain more about the words can, could, may , might
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Anonymoushere are the sentence;i can takr 100 calls a dayi could take 100 calls a day
 anonymous's reply was promoted to an answer.