1, different tone: "couldnt" is more polite than "cant".

2. Different meanings: "couldnt" is the abbreviation of "couldnt", which means "can't, can't", emphasizing the past facts and state, and "cant" is the abbreviation of "can't, can't", emphasizing the present and future facts and state.

3. Different usage: "couldnt" is followed by the prototype of the verb, indicating the speculation and assumption of the past situation, and "cant" is followed by the prototype of the verb, indicating the speculation and assumption of the present and future situation.