Modal verb: Must

Must (subjective obligation)

We often use must to say that something is essential or necessary, for example:

• I must go.

Structure of Must

Must is a modal auxiliary verb. It is followed by a main verb. The structure is:

subject + must + main verb

The main verb is the base verb (infinitive without "to").

Look at these examples:

subject	auxiliary must	main verb
I	must	go	home.
You	must	visit	us.
We	must	stop	now.

 

Like all auxiliary verbs, must CANNOT be followed by to. So, we say:

• I must go now. (not *I must to go now.)

Use of Must

In general, must expresses personal obligation. Must expresses what the speaker thinks is necessary. Must is subjective. Look at these examples:

• I must stop smoking.
• You must visit us soon.
• He must work harder.

In each of the above cases, the "obligation" is the opinion or idea of the person speaking. In fact, it is not a real obligation. It is not imposed from outside.

It is sometimes possible to use must for real obligation, for example a rule or a law. But generally we use have to for this.

We can use must to talk about the present or the future. Look at these examples:

• I must go now. (present)
• I must call my mother tomorrow. (future)

We cannot use must to talk about the past. We use have to to talk about the past.

From: EnglishClub.com