• We use must when we feel something is true.

EXAMPLE: You have been talking for so long your mouth must be very dry now.

• We use must when it is necessary to do something.

EXAMPLE: I haven't seen my parents for a long while. I must visit them at the weekend.

• We can use must for the present or future.

EXAMPLE: We must leave now before the weather gets worse.
EXAMPLE: We must leave very early tomorrow or we will be late.

• We use must have for something that we think happened in the past.

EXAMPLE: He must have gone out while I was sleeping.