Must (subjective obligation)
We often use must to say that something is essential or necessary, for example:
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:
Use of Must
In general, must expresses personal obligation. Must expresses what the speaker thinks is necessary. Must is subjective. Look at these examples:
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:
We cannot use must to talk about the past. We use have to to talk about the past.