Modal Verbs: May, Might, Must

4. May, Might and Must
One of the meanings of may and might is to be allowed to.
e.g. The members of the organization agree that I may join it.
      The members of the organization agreed that I might join it.

The auxiliary must is a stronger form of may and might. One of the meanings of must is to be obliged to or to have to.
e.g. You must provide proper identification in order to cash a check.
      They must work harder if they are to succeed.

It should be noted that the meaning of must not is to be obliged not to.
e.g. You must not leave.
      He must not speak.
The first example has the meaning, You must stay. The second example has the meaning, He must be silent.

In order to express the idea of not being obliged to do something, an expression such as not to be obliged to or not to have to is generally used.
e.g. You do not have to leave.
      He is not obliged to speak.
The first example has the meaning, You may stay, if you wish. The second example has the meaning, He may be silent, if he wishes.

Like could and would, might can be used in polite requests and suggestions. The auxiliaries could, would and might can be used to express differing degrees of politeness:

Degree of Politeness	Auxiliary
somewhat polite	could
quite polite	would
very polite	might

Thus, might expresses the highest degree of politeness.
e.g. Might I observe what you are doing?
      Might I offer some advice?

See Exercise 9.

May, might and must are also used to express differing degrees of probability:

Degree of Probability	Auxiliary
somewhat probable	may, might
highly probable	must

For instance, may and might are often used in the Simple conjugation to express the idea that an event is somewhat probable.
e.g. You might be right.
      It may snow later this afternoon.

Similarly, must can be used in the Simple conjugation to express the idea that an event is highly probable.
e.g. He must be mistaken.

In the following examples, the Perfect conjugations with may, might and must are used to express differing degrees of probability relating to past events.
      Rupert might have taken the money, but it seems unlikely.
      It is possible he may have called while we were out.
      It must have rained last night, because the streets are wet.

See Exercise 10.

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