Must or Musn´t

The verb "must" is also a modal verb, nor does infinitive (no "to must"). This modal verb has two main meanings. In its first meaning is used to indicate that we are sure that something is true.
I must-have gone. He should have gone (I'm sure he's gone)Inglés You Must speak very well. You must speak English very well (I'm sure)

The last form is built "must have past participle of main verb."
I Must Have His finished work. The work must be completedYou must-have won the match. You must have won the game

In its second meaning, "must" is used to indicate that something must be done, expressing an opinion of the speaker.
Must you answer the telephone. You must answer the phone (I understand that it is your duty)She Must study Inglés. She should learn English (this is my opinion)

In this second sense, "must" only be used in the present and the future.
You Must study today. You should study todayYou Must study next week. You should study next week

Denial is "must not" (or its abbreviated form "musn't"), which means you need to do something.
You musn't go there. You should not go thereI Must not sell historical house. He should not sell your home

And the question form is constructed by placing "must" at the beginning of the interrogation.
Must you water the garden? Need to get you water the garden?Must we play now? Should we play now?