Must is a 'modal auxiliary verb'. There is no -s in the third person singular.
He must start corning on hive. (NOT He musts . . .) Questions and negatives are made without do. Must you go? (not Do you must go?) You mustn't worry. (NOT You don't must worry.) After must, we use the infinitive without to. I must write to my mother. (NOT I must to write . . .)
Must has no infinitive or participles. When necessary, we use other expressions, such as have to.
He II have to start coming on time. (NOT He'll must . . .) i don't want to have to tell you again. (NOT I don't want to-must . . .)
Must has no past tense: We can talk about past obligation with had to.
I had to push the car to start it this morning. (NOT I must push . . .) Must can have a past meaning in reported speech. I told her she must be home by midnight.