Modal verbs

Modal verbs (⟦can⟧, ⟦could⟧, ⟦may⟧, ⟦might⟧, ⟦must⟧, ⟦should⟧, ⟦will⟧, ⟦would⟧) go before another verb to show ability, permission, possibility, advice or obligation. They never change: no ⟦-s⟧, no ⟦-ed⟧, and the verb after them is always the base form, without ⟦to⟧.

Key facts

  • Ability: ⟦I can swim.⟧ In the past: ⟦I could read when I was four.⟧
  • Permission: ⟦Can I go out?⟧ ⟦May I sit here?⟧ (more polite).
  • Possibility: ⟦It might rain.⟧ ⟦She may be late.⟧ ⟦That could be true.⟧
  • Advice: ⟦You should rest.⟧ ⟦You shouldn't shout.⟧
  • Obligation: ⟦You must wear a helmet.⟧ Not allowed: ⟦You mustn't run.⟧ Not necessary: ⟦You don't have to come.⟧
  • Form: ⟦She can swim.⟧ (not ⟦cans⟧, not ⟦can swims⟧, not ⟦can to swim⟧). Negative: ⟦can't⟧/⟦cannot⟧, ⟦won't⟧, ⟦shouldn't⟧, ⟦mustn't⟧.

Worked examples

Example 1

Choose: ⟦You have a fever. You (should / can) see a doctor.⟧

  1. The speaker gives advice: it is a good idea.
  2. Advice uses ⟦should⟧: ⟦You should see a doctor.⟧

Answer: ⟦should⟧

Example 2

Correct the mistake: ⟦He can sings very well.⟧

  1. After a modal verb the next verb is the base form.
  2. ⟦sings⟧ → ⟦sing⟧: ⟦He can sing very well.⟧

Answer: ⟦He can sing very well.⟧

Example 3

What is the difference? ⟦You mustn't go.⟧ / ⟦You don't have to go.⟧

  1. ⟦mustn't⟧: it is not allowed, so don't do it.
  2. ⟦don't have to⟧: it isn't necessary, but you can if you want.

Answer: not allowed / not necessary