CAN and MUST
CAN
CAN has only got a present (can) and past tense (could)
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So what do you use when the sentence is in the perfect or future tense?
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That depends on the meaning of CAN in the sentence.
CAN: ability or permission???
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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.
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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.