CAN and MUST
CAN
CAN has only got a present (can) and past tense (could)
So what do you use when the sentence is in the perfect or future tense?
That depends on the meaning of CAN in the sentence.
CAN: ability or permission???
I can fix your computer = ability (skills)
I was able to fix your computer yesterday/ I could fix your computer yesterday.
I have been able to fix the computer.
I will be able to fix your computer tomorrow.
I can come to your party = permission
I was allowed to come to your party last weekend/I could come to your party last .weekend
I have been allowed to come to your party.
I will be able to come to your party next week.
MUST
MUST has only got present tense. In other tenses HAVE TO is used.
I had to go to the dentist yesterday.
I have had to go to the dentist a lot lately.
I will have to go to the dentist tomorrow. (appointment)
When things are a little less necessary than MUST, you use NEED TO.
I think I need to call him one of these days.