Modal Auxiliaries generally express a speaker's attitudes or moods. For example, modals can express that a speaker feels something necessary, advisable, permissible, possible, or probable, and in addition, they can convey the strength of these attitude.
The modal auxiliaries in English are:
can, could, had better, may, might, must, ought to, shall, should, will, would.
Common expressions whose meanings are similar to those of some modals auxiliaries:
be able to = can
be going to = will
have to = must
have got to = must
be to = must
be supposed to = should
used to = would
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