MODALS VERBS

MODAL VERBS OF ADVICE
We use had better, ought to and should to give advice, make recommendations or say whatever we think is the right thing to do.
MODAL VERBS OF NECESSITY, OBLIGATION AND PROHIBITION

NECESSITYWe use need to to express necessity.

OBLIGATION
We use must or have to to talk about obligation.
We use needn't or don't have to when there is no obligation.

PROHIBITIONWe use mustn't to talk about prohibition..

MODALS OF ABILITY AND PERMISSION
ABILITY
We use can to talk about ability in the present
We use could to talk about ability in the past.
We use be able to to talk about ability in other tenses.
PERMISSION
We use can to talk about permission in the present.
We use be allowed to to talk about permission in other tenses.
MODALS OF POSSIBILY AND CERTAINTY
We use could, may and might to say something in possible.
We use can't or mustn't to say we are certain something is impossible.
We use must when we are certain something is true.