The necessity ( subjective meaning)

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6.2. The necessity ( subjective meaning)
The necessity of "must" is the speaker's supposition. This is the subjective meaning and similar to that of "may." But unlike it, this means the assertive supposition. (*21)

*21	This is not often used for the future because the assertive meaning is not parallel to the future which is not deterministic.

(8)
a. He left the office an hour ago. He must be at the airport by now.
b. If he says so, it must be true.

For example, (8a) can be changed into (9).

(9)
He left the office an hour ago. It is certain that he is at the airport by now.

If "may" is used, it is changed into (10a) and this can also be changed into (10b).

(10)
a. He left the office an hour ago. He may be at the airport by now.
b. He left the office an hour ago. It is possible that he is at the airport by now.

The different meaning can be seen since the adjectives, certain or possible, are used in each of them.

Now, this "may" is used in the negative sentence but this "must" is not usually used in it. "Can not" is used instead. The difference is seen as follows. (*22)

*22	In American English, it also tends to be used "mustn't." It is parallel to this "can't."

(11)
a. It mustn't be true.
(I think it is not true.)
b. It can't be true.
(I do not think it is true.)

These are derived from the negation of the modal auxiliary or the main verb. These are explained in 4.1 of the chapter 2.

This is the usage as mentioned above. It usually uses the stative verb. If the active verb is used, it usually expresses the obligation. To express the necessity with the active verb, it needs the progressive form. See the following example.

(12)
He must be working hard.

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