Suggestions

How do we usually make suggestions using modal verbs?
It's pretty easy.

Part 1

We use must when we're absolutely sure about something:

He has passed this difficult exam - he must be very clever!

We use could/may/might when we suppose that some statement is true:

It might/may/could rain tomorrow.

And finally, we use can't when we're absolutely sure that something isn't true:

I don't believe it - you can't be here!

! remember after modals we use only infinitives

Part 2
(more complicated part)

A little bit about different tenses:

Previous part was about Present Simple tense.

Other tenses:

Present Continious:
He is sleeping. - He must be sleeping.

Future Simple:
It will rain tomorrow. - It might rain tomorrow.

Past Simple:
He left. - He must have left.

And so on