A2 Elementary Modal verbs
Obligation: must, have to
Lesson goal. After this lesson you can say what is obligatory, what is forbidden and what is not necessary — three ideas that are easy to mix up.
When to use it
- must: you feel it is necessary — I must finish this tonight.
- have to: an outside rule — We have to wear a uniform.
- mustn't: it is forbidden — You mustn't use your phone in the exam.
- don't have to: it is optional — You don't have to come.
Form
| Form | Changes with subject | Example |
|---|---|---|
| must + base verb | no change | She must go. |
| have to / has to + verb | changes | She has to go. |
| mustn't + verb | no change | You mustn't go. |
| don't / doesn't have to + verb | changes | She doesn't have to go. |
| Questions with have to | Do / Does + S + have to…? | Do you have to work? |
Step by step
- Ask: is this obligatory, forbidden or unnecessary?
- Obligatory: outside rule → have to; your own sense of need → must.
- Forbidden → mustn't. Unnecessary → don't have to.
- Remember: have to changes with the subject (has to); must never does.
Two negatives, opposite meanings
| Sentence | Are you allowed? |
|---|---|
| You mustn't park here. | No — absolutely not. |
| You don't have to park here. | Yes — it's up to you. |
| You must wear a helmet. | You must do it. |
| You don't have to wear a tie. | It's up to you. |
Examples
Students have to wear a uniform.
I must call my mother tonight.
You mustn't touch that wire — it's dangerous.
We don't have to pay; the museum is free today.
Do you have to work at the weekend?
He has to wear glasses to drive.
Common mistakes
You mustn't come if you are busy. → You don't have to come if you are busy.
mustn't means forbidden. Here you mean it is not necessary → don't have to. This is the most serious meaning error in this lesson.
He must to finish it. → He must finish it.
No to after must. But have to always keeps its to — it is part of the phrase.
We don't must be late. → We mustn't be late.
There is no don't must. The negative of must is mustn't.
Memory tip
Use two questions: “Am I allowed?” No → mustn't. “Is it required?” No → don't have to. Different questions, different answers.
Summary
- must (your own sense) and have to (a rule) both mean obligation.
- mustn't = forbidden · don't have to = optional — opposite meanings.
- must never changes; have to becomes has to in the third person.
Practise this lesson
Do exercises with an explanation for every question, take mini tests and track your progress on GrammUp.