Modals of advice, necessity, obligation and prohibition

Must, have to, need:

We use must or have to ( which is not a modal verb) to talk about obligation.

• You must arrive on time.
• You have to look good on a date.

We use needn't and don't have to (which are not modal verbs) when there is no obligation.

• You needn't pay for everything on a date.
• We don't have to see each other every weekend.

The verb need (without to) is only used in the negative and interrogative. In the affirmative, we use need to to express necessity.

• You needn't call me later.
• I need to buy my boyfriend a present today.

We use mustn't to talk about prohibition.

• You mustn't cheat on your boy or girlfriend.

REMEMBER!

Try not to confuse must and don't have to. The meanings of must and have to are similar in the affirmative, but completely different in the negative. We use mustn't when there is strong prohibition and don't have to when there is no obligation.

• You have to call me this afternoon. (=You must ring me this afternoon.)
• You mustn't call me this afternoon. (=Don't call me.)
• You don't have to call me this afternoon. (=Is isn't necessay to call me.)

Ought to, should:

We use ought to and should to give advice and make recommendations.

• Yo ought to dress smartly.
• You should arrive on time.

Had better:

We use had better+infnitive to give advice or express an intention. It has a similar meaning to ought to and should.

• You'd better apologize to her.

We form the negative by putting not in front of the main verb.

• I feel terrible! I'd better not go out.

We can also use had better (not) in threats.

• You'd better not be late, or I'll be furious!