Obligation/Prohibition: 'must', 'have to'
A2 ElementaryLearn how to express obligation and prohibition using 'must' and 'have to', and how to describe a lack of necessity with 'don't have to'.
Overview
We use must and have to to talk about obligations—things that are necessary or required. While they are often interchangeable, must frequently expresses a personal obligation or a formal rule, whereas have to often refers to external rules or laws.
Negative Forms: Mustn't vs. Don't have to
It is important to note that mustn't and don't have to have completely different meanings. Use mustn't when something is against the rules, and don't have to when something is not necessary but still allowed.
| Form | Meaning | Example |
|---|---|---|
| mustn't | Prohibition (it is forbidden) | You mustn't smoke here. |
| don't have to | No obligation (it is optional) | You don't have to pay now. |
Form
must + infinitive / have to + infinitive
Must is a modal verb and does not change for the third person. Have to functions like a normal verb and changes to has to for he/she/it.
Examples
Common mistakes
He musts go now.
He must go now.
Modal verbs like 'must' do not take an -s in the third person singular.
I must to see a doctor.
I must see a doctor.
Do not use 'to' after 'must'. Use the bare infinitive.
You don't must park here.
You mustn't park here.
Use 'mustn't' for prohibition. 'Don't must' is grammatically incorrect.
Practice
The museum is free. You ___ pay to enter.
Show answer
don't have to
Answer:don't have to
Why:Since the museum is free, there is no obligation to pay, making it optional.
You ___ take photos here; it is strictly forbidden.
Show answer
mustn't
Answer:mustn't
Why:We use 'mustn't' to express prohibition or that something is against the rules.
John ___ wake up early every day for his new job.
Show answer
has to
Answer:has to
Why:We use 'has to' for third-person singular obligations imposed by external circumstances like a job.