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:

Like all auxiliary verbs, must CANNOT be followed by to. So, we say: I must go now. (not *I must to go now.)

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

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.

• 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.

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.

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