*Historically some of these modal
auxiliaries are in the present tense and others are in the past tense.
Present tense: can, shall, will, may
Past tense: could, should, would, might
In modern English, the past tense modal auxiliaries can all be used
to refer to the present or future, unlike other past tense forms (as in
He could be working now, I should work tomorrow, Mary might be at home
now, He would come tomorrow (if we let him)). In standard English,
the present tense modal auxiliaries cannot be used to refer to the past,
however, so I can work tomorrow is fine, but I can work yesterday
is awful.
Back to (3) What are the forms of the verb?