Modal verbs in short: can, must, have to, should

Can, must, should and have to show ability, necessity, advice or permission. Must and have to are often confused, because both can mean “I need to”, but they are used differently.

How they work and what they mean

The formula is simple: modal + base verb, with no to and no -s: I must go, Can you help?, You mustn’t stop. The exception is have to, which changes like an ordinary verb: he has to, does he have to?

• can — ability or permission: I can swim. For asking permission politely, see “Can I try this on?”
• should — advice, something sensible to do: You should rest.
• must — a strong necessity that the speaker feels: I must call my mom.
• have to — an obligation from outside, such as a rule, a timetable or a boss’s demand: I have to wear a uniform at work.

Must or have to: the main difference

Must shows what the speaker considers necessary: their own feeling or decision. Have to shows an obligation that comes from outside: a rule, a timetable, a doctor or a boss. In everyday talk about work, school and rules, have to is more common. The negatives are different too: mustn’t means “it’s forbidden”, while don’t have to means “it’s not necessary”.

• You can sit here if you want.Permission or possibility.
• You should drink more water.Advice: a good idea for your health.
• I must finish this today; it’s important.The speaker feels it is necessary.
• You mustn’t touch the paintings.It is forbidden.
• You don’t have to come tomorrow.It isn’t necessary; it’s your choice.
• Do you have to work on Sunday?Is it required by work or by the rules?

The most common mistake is an extra to after a modal verb. Say I must go, not I must to go, and don’t add -s: she must go, not she musts go. Have to does change: she has to go.

Check yourself

This is a short guide: the full explanation, all the uses of modal verbs and plenty of practice are in our lessons. Come to a free trial lesson, and meanwhile check yourself with our grammar quizzes.