Modals and related verb phrases add meanings to verbs. Below are some of those meanings:
1. Ability/Availability
· future: will be able to
· present: can, am/is/are able to
· past: could, was/were able to
2. Requests
· present/future: can, could, will, would
3. Permission
· future: will be allowed to
· present/future: may, can, could, am/is/are allowed to
· past: could, was/were allowed to
4. Possibility
· present/future: may, might, could
· past: may have, might have, could have
5. Impossibility
· present/future: couldn’t, can’t
· past: couldn’t have
6. Advisability
· present/future: should, ought to, had better
· past: should have, ought to have, had better have
7. Expectation
· present/future: should, ought to
· past: should have, ought to have
8. Necessity
· future: will have to
· present/future: must, have to, has to
· past: had to
9. Lack of Necessity
· future: won’t have to
· present/future: don’t have to, doesn’t have to
· past: didn’t have to
10. Prohibition
· present/future: must not, may not, cannot
· past: could not
11. Logical Deduction (=Probability)
· present: must, have to, has to
· past: must have, have to have, has to have