It has the same form of all the tenses, persons, numbers.
It is used----
a. To express compulsion or necessity.
b. To express duty or responsibility or a very strong obligation.
c. To express certainity or strong belief or very strong inference.
d. To express determination.
e. To express what cannot be avoided.
Note: Must refers to present or future but not to the past.
a. you must do it.
b. You must serve your parents.
c. He must return today.
d. I must join this college.
e. Die we must.