Modal Verb | Meaning | Expressing | Example |
must
| to have to | 100 % obligation | I must stop when the traffic lights turn red. |
to be very probable | logical conclusion (deduction) | He must be very tired after such enormous work |
must not
| not to be allowed to | prohibition | You must not smoke in the hospital. |
can
| to be able to | ability | I can swim |
to be allowed to | permission | Can I use your phone please? |
it is possible | possibility | Smoking can cause cancer ! |
could
| to be able to | ability in the past | When I was younger I could stay up all night and not get tired.. |
to be allowed to | more polite permission | Excuse me, could I just say something? |
it is possible | possibility | It could rain tomorrow! |
may
| to be allowed to | permission | May I use your phone please? |
it is possible, probable | possibility, probability | It may rain tomorrow! |
might
| to be allowed to | more polite permission | Might I use your phone please? |
it is possible, probable | weak possibility, probability | I might come and visit you in America next year, if I can save enough money. |
need
| necessary | necessity | Need I say more? |
need not
| not necessary | lack of necessity/absence of obligation | I need not buy any tomatoes. There are plenty in the fridge. |
should/ought to
| used to say or ask what is the correct or best thing to do | 50 % obligation | I should / ought to see a doctor. I have a terrible headache. |
to suggest an action or to show that it is necessary | advice | You should / ought to revise your lessons |
to be very probable | logical conclusion (deduction) | He should / ought to be very tired after such enormous work |
had better
| to suggest an action or to show that it is necessary | advice | You 'd better revise your lessons |