Here you will find an explanation on the use of Modals and the different Modal Verbs

Modal Exercise 1 can, could, have to, must, might and should
Modal Exercise 2 have to and must
Modal Exercise 3 might, must, should, could, have to and ought to
Modal Exercise 4 couldn’t and might not
Modal Exercise 5 have got to, had better, may and shall
Modal Exercise 6 could, might, should and would
Modal Exercise 7 modal verb forms
Modal Verb Final Test complete review