Obligation, prohibition, necessity (advanced)
B1 IntermediateLearn how to express obligation, necessity, and prohibition using modal verbs and semi-modals. This guide covers the functional differences between must, have to, and should at an intermediate level.
Overview
English uses different verbs to show how necessary an action is. Must and have to express strong obligation, while should expresses advice or a weak obligation. In the negative, mustn't indicates a prohibition, whereas don't have to indicates a lack of necessity.
Must vs. Have to
Note that must is rarely used in the past tense for obligation; use had to instead.
| Verb | Usage | Example |
|---|---|---|
| must | Internal obligation; the speaker feels it is necessary. | I must finish this report tonight. |
| have to | External obligation; rules, laws, or outside circumstances. | I have to wear a uniform at work. |
| mustn't | Prohibition; it is forbidden to do something. | You mustn't smoke in here. |
| don't have to | No necessity; you can do it, but it is not required. | You don't have to come if you are busy. |
Form
Subject + modal + base verb (infinitive without 'to')
Modal verbs like must and should do not change for the third person singular. Have to is a semi-modal and follows standard conjugation rules.
Common mistakes
I must to go now.
I must go now.
Must is a modal verb and is followed by the bare infinitive without 'to'.
You mustn't bring a gift, but you can if you want.
You don't have to bring a gift, but you can if you want.
Mustn't means it is forbidden. Use 'don't have to' when an action is optional.
Practice
The sign says 'No Entry'. You ___ enter this room.
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mustn't
Answer:mustn't
Why:Mustn't is used for prohibitions or actions that are against the rules.
It's a holiday tomorrow, so I ___ go to work.
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don't have to
Answer:don't have to
Why:Don't have to expresses a lack of necessity; there is no obligation to work on a holiday.
Yesterday, the elevator was broken, so I ___ take the stairs.
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had to
Answer:had to
Why:Had to is the past tense form of both 'must' and 'have to' for obligation.