Verbs such as can, will, should, must, need to and be
allowed to are examples of modal verbs.
Modal verbs are used with another verb. The second verb will be in the
infinitive form (first form).
I can speak
I am allowed
to leave early
I should be
Modals can be used to express intention (will, should), obligation (must, have
to, should, ought to), ability, (can, was able to), or permission (am allowed
to, can, may).
Modals are 'imperfect' verbs, they do not decline like other verbs. The
past form of ‘must’ is ‘had to’; the future form of ‘can’ is ‘will be able to’.