English grammar: Modal verbs for obligation

What this grammar point covers

Modal verbs for obligation are special verbs in English (like must, have to, should) that we use to talk about things that are necessary or required.

When to use it

Use these modal verbs to talk about rules, duties, or things that are important or required. 'Must' and 'have to' show strong obligation, while 'should' is for advice or weaker obligation.

Key forms

• must + base verb (e.g. must go)
• have to + base verb (e.g. have to study)
• should + base verb (e.g. should wear)

Examples

Tips

• 'Must' and 'have to' are often similar, but 'have to' is more common for rules from outside (like laws).
• 'Should' is not as strong as 'must' or 'have to'.
• Don't use 'to' after 'must' or 'should'.

Exceptions and edge cases

• For the past, use 'had to' instead of 'must'.
• 'Must' does not have an infinitive or past form (don't say 'to must' or 'musted').