We use modal verbs when:
a.) we talk abort the possibility of something:
-must - when we are sure that something is true: It must be John.
-could be - when something is possible but improbable: It could be John.
-might be - when something is possible but improbable: It might be John.
-can't be - when something is impossible: It can't be John.
b.) we talk about the possibility of something happening in the past:
-must have been - we are sure that something happened: He must have been here yesterday, he left his shoes.
-could have been - we think that maybe something happened: He could have been here yesterday,
-might have been - we think that maybe something happened: : He might have been here yesterday
-can't have been - we believe that something did not happen: He can't have been here yesterday - it is too clean in here.
c.) we use modal verb with gerundium (verb+ing)
-It must be raining (we are sure that it is)
-You might try again (suggestion)
d.) instead of might or could we can use may to express the possibility of something:
-They may have left already (possibility in the past)
-We may go to London (possibility)
e.) we use must and can't to when we deduce something. We use Must when we there are signs that something is true and can't when something is impossible.
-I hear baking - there must be a dog in the house
-It can't be Jack waiting at the door; he died 5 years ago!
f.) may/might/could express the possibility of something happening in the present of in the future
-Do not enter the room! It might be a trap!
-I may be late because of the traffic
-there is nobody at home. They could go to work
g.) we use will when we predict that something will happen because it happened in the past many times (it is a routine):
-I smell something terrible. It will be John. He always over fries the sausage.
