Grammar

Modal verbs must, have to and should

1,380 fill-in-the-gap exercises across 3 forms

Three ways to talk about obligation, and one pair of negatives that means opposite things.

Pick a form to practice

The three in one line

• must — the obligation comes from the speaker. I must stop smoking.
• have to — the obligation comes from outside: a rule, a law, a boss. I have to wear a uniform.
• should — not an obligation at all, but advice. You should see a doctor.

In practice the first two overlap constantly, and in American English have to has largely taken over. The distinction still shows up in tests, so it is worth holding onto.

The negatives are not a pair

This is the most important thing on this page, and the place most learners lose marks.

They are not two ways of saying the same thing. They are close to opposites.

Shouldn’t sits between them: You shouldn’t tell him means it would be unwise.

have to behaves like an ordinary verb

Must and should are modals: no -s, no to, no do. Have to is not a modal at all, and it follows the ordinary rules:

• it changes for the subject: he has to
• it needs do for negatives and questions: Do you have to go? He doesn’t have to go.
• it has a past and a future: I had to go. I will have to go.

That last point matters, because must has no past. For a past obligation, English uses had to:

I had to work late. — never I musted.

must for deduction

Must has a second job that has nothing to do with obligation: a confident conclusion.

The lights are on — they must be home.

Its negative here is not mustn’t but can’t: They can’t be home; the car has gone.

For the past, use must have and can’t have: She must have forgotten.

All forms

Mistakes to avoid