Modal verbs

➡️ MUST :
- when you
speculate about the truth of something and you think something is true:
It must be difficult.
He must be older than 50.
- to express obligation:
You must be more careful, John!
We must do all we can to help them.
➡️ HAVE TO is also used to express obligation
- you know you must do something you don't want to do but it is not an order (required by law for example).
She has to take care of Alice because her mother is on holidays.
- When the sentence is not in the present tense, you must use "have to" instead of "must":
My shirt will have to be ironed.
He had to wait for them a very long time.
➡️ MUSTN'T
to forbid someone to do something (almost an order).
You mustn't walk on that bridge: it's far too dangerous.
➡️ MAY:
you don't know if that event will occur:
He may become famous one day.
It may happen again.
Can
be used for permission:
Yes, darling, you may have another biscuit.
➡️ CAN'T:
- you can't do something because you don't know how to do it/it is not allowed:
She can't walk, she's had an accident.
I can't work in the dark, I don't see well enough.
- not logical / not possible:
She can't be your sister! I don't believe it!
It can't be six o'clock already.
I can't watch TV tonight, the TV set is out of order.
- forbidden actions (rules, laws):
You can't park here!
➡️ SHOULD :
- for advice/suggestions:
You should work more if you want to pass your exam.