Modal Verbs and their substitutes

Modal verbs are for example may, can, must, should, need. They express an ability, permission, wish etc. to do something. (I may, can, must swim.) Many modal verbs cannot be used in all of the English tenses. That's why we need to know the substitutes to these modal verbs.

Modal Verb	Substitute	Example
must	to have to	I must swim. = I have to swim.
must not	not to be allowed to	I must not swim. = I am not allowed to swim.
can	to be able to	I can swim. = I am able to swim.
may	to be allowed to	I may swim. = I am allowed to swim.
need	to have to	I need to swim. = I have to swim.
need not	not to have to	I need not swim. = I don't have to swim.
shall / should/ ought to	to be supposed to / to be expected to / to be to	I shall / should / ought to swim. = I am supposed to swim. / I am expected to swim. / I am to swim.