Modal Verbs

Form

A modal verb sits straight in front of the base verb and never changes its own form (she can swim, he must leave — no -s, and no to).

Questions swap the modal and the subject (Can you swim?), and negatives add not after it (can't, mustn't, shouldn't). To talk about the past, most modals take modal + have + the past participle (She must have forgotten; You should have called). Can, must and may run out of forms, so English borrows be able to, have to and be allowed to for the tenses they can't make (I will be able to drive; We had to wait).

When to Use It

1. 1

   Ability, with can and could

   She can speak three languages.

2. 2

   Permission and polite requests, with can, could and may

   May I use your phone?

3. 3

   Obligation and prohibition, with must, have to and mustn't

   You must wear a seatbelt.

4. 4

   Advice, with should and ought to

   You should see a dentist about that tooth.

5. 5

   How sure you are, with must, may, might and can't

   He has lived in Rome for years, so he must speak Italian.

6. 6

   Looking back at the past, with modal + have + participle

   You should have told me you were coming.

Common Mistakes

• To after a modal: write I can help you, not I can to help you — a modal is always followed by the base verb.
• An -s in the third person: write She must leave now, not She musts leave now — modals have one form for every subject.
• Mustn't for something optional: write You don't have to bring a gift, not You mustn't bring a gift — mustn't forbids it, while don't have to means it simply isn't necessary.
• Two modals in a row: write You will be able to park here, not You will can park here — only one modal per verb phrase.
• Must for the past: write I had to wait an hour, not I must waited an hour — must has no past form, so have to fills in.