We can use ‘must’ to express a number of different things:
- Expressing obligation or necessity;
- Saying something is prohibited;
- Expressing certainty.
Expressing Obligation or Necessity
We can use ‘must’ to say something is required or necessary.
You must have a licence to drive a car.You are required to have a licence before you can drive. If you don’t have a licence, you can’t drive.
I must remember to buy a birthday present for my sister.It is important I buy a present. I need to remember it.
I must not forget to buy a birthday present for my sister.It is important I buy a present. I need to remember it.
Saying Something is Prohibited
We use ‘must not’ or ‘mustn’t’ to say something is prohibited, forbidden, or not allowed.
Guests mustn’t smoke in their hotel room.Guests are not allowed to smoke in their room. It is forbidden.
You must not ride your bicycle on the grass.You can’t ride your bicycle on the grass. It is prohibited.
We can use ‘must’ when we are certain something is true. We are certain because we have evidence, or the result is logical.
You must be tired after your long journey.I am certain you are tired. I believe you are tired because long journeys are usually tiring.
We must have gone the wrong way.We are lost. The logical reason is that we went the wrong way.
Making Sentences Using Must
+ I must leave. s=I + must + v
- I must not leave. s=I + must + not + v
? Must I leave? must + s=I + v