Grammar: ⚙️ Verbs in Depth: 🎭 Modals & 🌱 Non-Finite Verbs (Advanced) (B2) - Lesson 2: Modals of Deduction & Speculation - Past (must have been, might have V3, couldn't have V3)

✨ Lesson 2: Modals of Deduction & Speculation (Past)

In our last lesson, we learned to make logical guesses about the present. Today, we will use a similar skill to make a past deduction¹. We look at the evidence² we have now and make a logical guess about what happened in the past to cause the present situation.

The Structure: Modal + have + Past Participle

To make a deduction about the past, we use a specific structure. The modal verb shows our certainty, and the "have + past participle" form pushes the action into the past.

~95% Sure (Strong Deduction): use must have + p.p.

Use this when you have strong evidence and you are making a logical conclusion about the past.

"The ground is wet this morning. It must have rained last night."

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~50% Sure (Possibility): use might have / may have / could have + p.p.

Use these when a past event was possible, but you are not sure.

"Sothea is late for the meeting. She might have gotten stuck in traffic."

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~5% Sure (Almost Impossible³): use can't have / couldn't have + p.p.

Use these when you are almost certain something did NOT happen.

"He looks happy and relaxed. He can't have failed his exam."

Important Note on Negatives

This is a very common point of confusion. To make a negative deduction about the past, we use 'can't have' or 'couldn't have'. We do NOT use 'mustn't have'.

Prohibition⁴ (Rule): "You mustn't smoke here."

Past Deduction (Impossibility): "She can't have known about the party; if she had, she would have come."

We use 'might not have' to say something possibly didn't happen: "He's not here yet. He might not have received my message."

🧠 Practice Quiz: What Happened?

Choose the best modal form to complete each sentence.

1. I can't find my wallet anywhere! I _______ it at the restaurant.
   Answer: must have left (This is the most logical conclusion.)
2. She didn't answer her phone when I called. She _______ busy.
   Answer: might have been (This is one possibility.)
3. He passed the B2 exam with a perfect score. He _______ studied very hard.
   Answer: must have
4. You say you saw him in Battambang this morning? Then you _______ seen him in Phnom Penh last night. It's too far to travel that fast.
   Answer: can't have (It's logically impossible.)
5. A: "Why didn't Dara come to the party?" B: "I'm not sure. He _______ about it."
   Answer: might have forgotten (This is a possible reason.)

📝 Homework: Past Detective

Look at the present evidence and write a sentence of deduction about the past using 'must have', 'might have', or 'can't have'.

1. Evidence: The window is broken and a rock is on the floor inside.
   (Example: Someone must have thrown a rock through the window.)
   _________________________________________
2. Evidence: Your friend looks very happy and excited today.
   (Example: She might have received some good news.)
   _________________________________________
3. Evidence: The cake is all gone, but the plates are still on the table.
   (Example: They can't have finished cleaning up yet.)
   _________________________________________

Vocabulary Glossary

1. Past Deduction: (Noun Phrase) - ការសន្និដ្ឋានអំពីអតីតកាល (kaa sân'nĭ'tʰaan âm'pi â'ti'tâ'kal) - A logical guess or conclusion about a past event based on present evidence. ↩
2. Evidence: (Noun) - ភស្តុតាង (phós'dŏ'tang) - The available facts or information indicating whether a belief is true or valid. ↩
3. Impossible: (Adjective) - ដែលមិនអាចទៅរួច (dael mĭn aach tov ruəch) - Not able to occur, exist, or be done. ↩
4. Prohibition: (Noun) - បម្រាម (bâm'ram) - The action of forbidding something; a rule against something. ↩
5. Speculate: (Verb) - ស្មាន (s'maan) - To form a theory or guess about a subject without firm evidence. ↩