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.
|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.|