Modals of speculation are used to express degree of certainity. We use them to describe something that is possible, probable, definite, impossible in the speaker’s opinion.
The sentence structure is:
Subject + modal verb + main verb + other parts of the sentence
The main verb is used in the infinitive form, when we talk about the future.
We use must when we are sure that something is true, and can’t when we are sure that something is not true.
They must be good workers.
It can’t be your car!
We use might, may, could when we suppose it’s possible that something is true. May/may not are more certain than might/might not, could.
It may not be bad idea.
He might have done his homework.
This dress could be very expensive.
We use modal verbs with Perfect Infinitive to express the possibility of an action in the past.
She must have gone to school already.
I don’t think he can have done this.
They may not have seen these photos.