Past deductions: must have, might have and can’t have

The idea

Use modal + have + past participle: must have moved, may have left. Use can't/couldn't have to reject a past explanation confidently. For a possibly ongoing past activity, use might have been waiting. Have stays unchanged after the modal.

Examples

• Someone must have watered the plants; the soil is wet.
• I may have left my notes on the bus.
• This can't have been the original colour; the old photograph shows a red door.
• They might have been waiting at the other entrance.
• Someone must have moved the ladder; it isn't by the shed.
• Nora may not have seen the updated timetable.

Watch out

Write must have, not must of or must has. In these examples must have draws a conclusion; ordinary past obligation uses had to. May not have seen means possibly did not see, whereas can't have seen rejects having seen it. Inference does not make either claim a verified fact.

Try it yourself

1. Choose the correct form. Someone must ___ watered the plants. (have / has)

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   have

   After a modal, use unchanged have, even with singular someone.

2. Choose the correct form. This can't have ___ the original colour. (been / was)

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   been

   The past participle of be is been, so the pattern is can't have been.

3. Say it is possible Nora did not see it; you are not sure. Nora ___ have seen it. (may not / can't)

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   may not

   May not have leaves the negative possibility uncertain. Can't have would express a confident rejection.

Use it today

Choose two traces of earlier activity, such as wet soil or a moved object. Give one confident and one uncertain explanation. Add a possible negative explanation with may not have, then state what evidence would confirm it.