Intermediate 📐 Grammar · #275 of 305

Certainty Scale

must > should > may/might/could > can't.

Examples

  • He must be at home.
  • He can't be at home.

Key rules

  • Must is about 90 percent sure: He must be at home
  • Should shows expectation: She should be here soon
  • May, might, could show about 50 percent: He might be late
  • Can't shows the opposite of must

Common mistakes

  • He must to be at home → He must be at home
  • He mustn't be at home → He can't be at home
💡 Tip: Use can't, not mustn't, for logical negatives.
Practise this topic in the appPractise Intermediate quiz
← Modals of Possibility: Could, May, Might, Can'tVerb + Gerund or Infinitive: Stop, Remember, Forget →

Related Intermediate topics

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Present Perfect Continuous

have been + -ing stresses duration up to now.

Past Perfect

had + past participle for an earlier past action.

Past Perfect Continuous

had been + -ing for duration before a past moment.

Future Continuous

will be + -ing for actions in progress at a future time.

Future Perfect

will have + past participle for completion before a future time.

Future Perfect Continuous

will have been + -ing for duration up to a future point.