14. MODAL VERBS
| POSITIVE | NEGATIVE | QUESTION |
| I have to (I don’t have to) | I should (I shouldn’t) | I can (I can’t) |
| I must (I mustn’t) | I ought to (I oughtn’t to) | I could (I couldn’t) |
| I need to (I don’t need to/needn’t) | I had better (I had better not) | I might / may (I might not, I may not) |
USAGE :
Modal verbs are used to express possibility, and weak or strong obligation. Modal verbs do not take ‘s’ in the third person, and do not have past or future forms*.
*Except have to and need to
EXAMPLES :
I have to go, I’ve got a conference call at 11.
You should confirm the appointment by email before you go.
I might go skiing this weekend.
You can go in, you don’t need to wait.
You ought to see a doctor about that cough.
NOTE : MUSTN’T / DON’T HAVE TO
Have to and must are the same in positive forms (obligation). However in the negative the meaning is different: mustn’t means it is prohibited (like can’t), but don’t have to means it is not necessary (like needn’t).
You mustn’t smoke in here, it’s dangerous / In my company we don’t have to wear a tie.