Modal Verbs

WILL + root verb
*for the unarranged / unplanned future (previous plans use "going to")
*for something that is bound to happen if a condition is met
*for offers or quick decisions
*for events that (we think, know, fear or guess) will happen in the future
CAN + root verb
*when you know how to do something or for something that is possible
CAN’T + root verb
*for things that are impossible or for a negative logical deduction
COULD + root verb
*for the same things, but referred to the past
BE ABLE TO + root verb
*for other tenses
MUST + root verb
*when we think it is necessary to do something
*for a logical deduction
MUSTN’T + root verb
*when it is forbidden to do something
NEEDN’T + root verb
*when something isn’t necessary
HAVE TO + root verb
*for other tenses
SHOULD + root verb
*when the action is the advisable (right) thing to do
MAY / MIGHT + root verb
*when something is quite possible
CAN’T HAVE + p. p.
*for a negative logical deduction in the past
COULD HAVE + p. p.
*for a thing that didn’t happen
although it would have been possible
ST HAVE + p. p.
*for a logical deduction on something that happened in the past
NEEDN’T HAVE + p. p.
*when something happened although it wasn’t necessary
SHOULD HAVE + p. p.
*when the advisable (right) thing to do didn’t take place
MAY HAVE + p. p.
*when it is quite possible that something happened

Please note: the root verb is the infinitive without “to”; p. p. is the past participle