We use the modal verbs: must, should, could, will, can (can’t), have to, might and may + a bare form (without: "to") when we think something is possible or sure about a situation happened in the past, present or future- all previous modals can be used into negative form depending on the idea you want to express.
We use must/ have to when we feel sure that something is true because there’s very strong evidence.
-Situation: He has come to the school on foot.
a. He must have had a problem with his car.
b. He has to be trying to lose weight.
We use might, may, will, can, should or could to say that we think something is possible but we’re not sure.
-Situation: He played too bad. He looked so tired!
a. He might have had an exam earlier.
b. He may have some health problems.
c. He will be punished by the coach. ("Will" is used when you talk about a more certain possibility because you know specific information about the situation)
d. He can be sick.
e. He should see a doctor as soon as possible. ("Should" is used when you want something to happen)
f. He could eat something rotten.
We use can’t when we feel sure about something is not true.
-Situation: Did you hear that noise in the yard?
a. It can’t be a thief. The alarm is working.
We use may when we offer a possible explanation in the present.
-Situation: He is not answering my calls.
a. He may be busy.
We use might when we offer a possible explanation in the future.
Situation: She might be angry with me since I haven’t sent her my part of the project.