MODALS OF OBLIGATION
MUST
Form: Subject + MUST + infinitive verb: I must clean my room.
It is used in the PRESENT tense.
HAVE TO
Form: Subject + HAVE (in the right tense) + TO + infinitive verb: She has to buy
food.
HAVE TO can be used in any verb tense (present, past or future):
-He has to tidy his bedroom (present)
-You had to do your homework (past)
-They will have to bring the books (future)
There’s not much difference between MUST and HAVE TO, we just have to be
careful with the verb tenses.
MUSTN’T / CAN’T
Form: Subject + MUSTN’T / CAN`T + infinitive verb: I mustn’t smoke here / You
can’t use your phone in class.
Use: They are used when something is not allowed.
They are used in the PRESENT tense.
DON’T HAVE TO
Form: Subject + DON`T HAVE (or any other tense) + TO + infinitive verb: You
don’t have to make dinner / He didn’t have to do the activity.
Use: it is used when something is not necessary.
SHOULD / OUGHT TO
Form: Subject + OUGHT TO / SHOULD + infinitive verb: She should go to the
doctor / I ought to come to class.
Use: to give advice or recommendation.