Modal Verb of Obligation

ALULA staff, Updated on August 02, 2023

  • Modal verbs are auxiliary verbs that express possibility, permission, necessity, etc.
  • Modal verbs of obligation express something that must be done usually because of external factors like laws or rules.
  • 'Must', 'have to' and 'should' are the most common modal verbs of obligation.
  • The context or the speaker’s reason for making the obligation important differ the usage of 'must', 'have to' and 'should'.
  • In negative forms, 'must not' indicates prohibition, 'do not have to' refers to lack of necessity, while 'should not' advises against something.

What are modal verbs?

Modal verbs are a type of auxiliary verbs that express a variety of meanings, such as possibility, permission, ability, and most importantly, necessity or obligation. Unlike regular verbs, modal verbs do not change form in terms of tense or person, and they are not used alone but together with another verb.

Modal verbs of obligation

Modal verbs of obligation express something that must be done, often because of a rule or law, or because it's necessary. The most common modal verbs of obligation are 'must', 'have to' and 'should'.


  • You must stop when the traffic lights are red.

    Use of 'must' to express a law or rule.

  • I have to finish this project by tomorrow.

    Use of 'have to' to express personal obligation.

  • You should study for the test on Friday.

    Use of 'should' to express advice or recommendation.

Differences in usage

Despite their similarities, there are subtle differences in the usage of 'must', 'have to' and 'should' based on the context or the speaker’s reason for making the obligation. 'Must' typically indicates an obligation that is imposed by the speaker, while 'have to' usually refers to an obligation imposed by an external rule or circumstance. On the other hand, 'should' is used to give advice or suggestions.


  • You must be here by 8:00. (as a boss to an employee)

    Use of 'must' to impose an obligation by the speaker.

  • I have to be at the airport two hours before my flight.

    Use of 'have to' for an external obligation.

  • You should drink plenty of water during hot weather.

    Use of 'should' for giving advice.

Negative form of obligation modals

In their negative forms, these verbs also have different meanings. 'Must not' is used to indicate something is prohibited, 'do not have to' refers to something that isn't necessary, while 'should not' is used to advise against something.


  • You must not use your phone while driving.

    Use of 'must not' for prohibition.

  • We do not have to wear uniforms in this school.

    Use of 'do not have to' indicative of lack of necessity.

  • You should not cheat on your taxes.

    Use of 'should not' for advising against an action.

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