Modal Auxiliary Verbs.

Modal auxiliary verbs never change form. You cannot add an -ed, -ing, or -s ending to these words. Can, could, may, might, must, ought to, shall, should, will, and would have only one form.

You can use modal auxiliary verbs in these patterns:

MODAL + MAIN VERB

MODAL + BE + PRESENT PARTICIPLE

MODAL + HAVE + PAST PARTICIPLE

With modal auxiliaries, you can indicate necessity or obligation:

To lose her orange glow, Yvonne should eat fewer carrots.

John must remember his wife's birthday this year.

If Cecilia wants a nice lawn, she ought to be raking the leaves.

Or you can show possibility:

Fred might share his calculus homework if you offer him a slice of pizza.

Ann could have run the half marathon if she had started to train four months ago.

Modal auxiliaries also show willingness or ability:

Nicole will babysit your pet iguana for a reasonable fee.

Jason can pass chemistry this semester if he stops spending his study time at the arcade.