00 / Modal Verbs
Obligation & necessity (advanced)
Learn 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.
B1Intermediate
01 / 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.
02 / 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. |
03 / 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.
04 / 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.
05 / Practice
The sign says 'No Entry'. You ___ enter this room.
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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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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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Answer: had to
Why: Had to is the past tense form of both 'must' and 'have to' for obligation.