Modal verbs.

could / may / might

Examples:

It might rain tomorrow, I am not sure.

She may be late for the movie

They could come by car.

must / can't / couldn't

Examples:

You worked all day. You must be tired.

She can't be sick. I' was just with her.

That couldn't be true. I trust her.

may not / might not

Examples:

She may not come to the party. They had a fight yesterday.

I might not have turned the lights off. I can't remember!

subject + modal verb + verb in the base form + complement.

Example:

They must be very happy.

v/s

I am sure they are over the moon.

subject + modal verb + be + verb -ing + complement.

I might be getting my wisdom tooth out.

v/s

I am probably getting my wisdom tooth out.

subject + modal verb + have + verb in the past participle + complement.

He may have felt ill.

v/s

He probably felt ill.