Must, Have To and Should: Obligation and Advice

Modal verbs help us say how necessary, advisable or possible something is. Must, have to and should are the core of obligation and advice.

Must: personal obligation

Use must when the obligation comes from the speaker's own feelings or strong opinion, or in official written rules.

• I must call my mother tonight. (I feel it is necessary.)
• Passengers must wear seat belts. (A written rule.)

Have to: external obligation

Use have to when someone else or the situation creates the obligation.

• I have to wear a uniform at work.
• She has to take the bus because her car is broken.

Unlike must, have to has a past and future form: "I had to leave early", "You will have to pay."

The important negatives

Form	Meaning	Example
mustn't	It is forbidden	You mustn't smoke here.
don't have to	It is not necessary	You don't have to come, but you can.
shouldn't	It is not a good idea	You shouldn't eat so much sugar.

This is the biggest trap: "You mustn't go" means you are not allowed, while "You don't have to go" means you may stay home.

Should and ought to: advice

Should gives advice or an opinion: "You should see a doctor." It is weaker than must. "Ought to" has nearly the same meaning but is less common.

Related modals

• Can / could: ability and permission. For one specific achievement in the past, use "was able to": "I was able to finish the race."
• May / might: possibility. "It might rain" is less sure than "It may rain", and both are less sure than "It will rain."

Common mistakes

• Wrong: "He musts go." Right: "He must go" (no -s after modals).
• Wrong: "I must to study." Right: "I must study." (Modals take the bare infinitive.)
• Wrong: "You don't must wait." Right: "You mustn't wait" or "You don't have to wait", depending on the meaning.

Practice

• Must, Have to or Should? quiz — Strong obligation, external rule, or just advice?
• May, Might or Will? quiz — Certain or uncertain about the future?
• Can, Could or Was Able To? quiz — General ability, past ability, or a specific achievement?