What you'll learn
By the end of this lesson, you'll be able to use "must have + past participle" to make strong deductions about the past.
When do I use it?
Use "must have + past participle" when the evidence tells you something almost certainly happened in the past:
- "The ground is wet — it must have rained."
- "He must have forgotten the meeting. He's not here."
It is the past version of "must" (near-certainty): "must rain" → about now; "must have rained" → about the past.
How do I form it?
subject + must have + past participle
- She must have worked late last night.
- They must have missed the train.
- You must have been tired after the flight.
- It must have cost a fortune.
Common mistakes for Spanish speakers
Spanish "debe de haber sido" maps to "must have been", but speakers often build it with the wrong infinitive: "must to have", "must has gone", or "must had gone". Keep the fixed formula: must + have + past participle — no "to", and the participle never changes with the subject. "No debe de haber sido" is "mustn't have been" only when you mean you deduce it didn't happen — but for impossibility English prefers "can't have been".
must + have + past participle — a sure guess about the past.
Practice
Multiple Choice
B2Which sentence is correct?
Fill in the Blank
B2You look tired — you must been working all night.
Hint: must + have + past participle.
Find the Error
B2Sentence Transformation
B2Task: Rewrite with must have + participle
Hint: A sure deduction about the past → must have + participle.
Practice Speaking
Apply what you've learned in a real conversation with AI feedback.
Level: B2