Modal Auxiliaries (mod)

Problems with modal auxiliaries (words like "can," "should," "have to,"etc.) can be divided into problems of form and problems of meaning.  Some of the most common problems of form are:

    1.  *He can understands.  (Use the simple form after a modal.)
    2.  *He can to understand.  (Most modals do not use "to")
    3.  *He may work yesterday.  ("may have worked"-- modals
          have their own way of forming the past tense.)
    4.  *He said he will go but he didn't.  ("would go"-- in past
          situations some modals change form.)

The wide variety of meanings expressed by modals is even more difficult.  The first link below summarizes the meanings and forms of modals; the next two give exercises.

Links:

    Meanings and forms:  modals.html
     Exercises:  modalintro.html
     Another exercise:  modals
     Perfect modals:  perfect modals
     More perfects: perfect modals