Modal Verbs: Must

The Forms, Meanings and Use of the Modal Verb MUST.

Must is a modal auxiliary verb. 'Must' has no infinitive or participles. It  has only one tense form in the present. Like other modal verbs, 'must' has no –s on the third person singular (he must, not he musts); questions and negatives are made without do (Must I?, not Do I must?); 'must' is followed by the infinitive without to (e. g.: I must do it).

Patterns. Read and memorize! 

Affirmative	Interrogative	Negative
I must do it now.	Must I do it now?	I must not do it now.
He must do it now.	Must he do it now?	He must not do it now.
She must do it now.	Must she do it now?	She must not do it now.
It must do it now.	Must it do it now?	It must not do it now.
We must do it now.	Must we do it now?	We must not do it now.
You must do it now.	Must you do it now?	You must not do it now.
They must do it now.	Must they do it now?	They must not do it now.

Note 1: - The missing tenses are supplied by the past and the future tenses of to have.
E. g. : I had to meet him at the station.
          I shall have to meet him at the station.

Note 2: - There is a contracted negative form mustn’t [mʌsnt].
E. g.: He mustn’t find out what’s happening.