Could Not Couldn't Grammar?

English-U.S. "Couldn't" is just a contraction of "could not." They mean the same thing. "She mustn't (must not) have seen you" is completely grammatical. Again, "mustn't" is just the contraction of "must not." No difference in meaning.

Can Can T could couldn't grammar?

COULD and COULDN'T are the past tense forms of CAN and CAN'T. COULD and COULDN'T refers to ability or inability in the past. ... We use COULDN'T to mean that we weren't able to do something in the past. Ex: When she was 1, she couldn't speak.

Can't or could not?

In statements, we use "can't" for present tense, and "couldn't" for past tense. ... "Couldn't" is used to refer to impossible things in the past. So, we cannot use them interchangeably. So, present tense, "can't," past tense, "couldn't."