Grammar: Modals of Deduction
B2 Lesson 2: Speculating About the Past
Why It Matters: From Evidence to Deduction
This grammar allows you to connect present evidence to the past. You can look at a current situation and make a logical guess about what caused it.
The ground is wet this morning.
It must have rained last night.
The Grammar Rule 📖
To make a deduction about the past, we use a specific structure that shows our level of certainty.
Structure for Past Deductions
| Certainty | Modal Verb | Example |
|---|---|---|
| ~95% Sure (Strong Deduction) | must have | The lights are on. They must have arrived home. |
| ~50% Sure (Possibility) | might have may have could have | Sothea is late. She might have gotten stuck in traffic. |
| ~5% Sure (Almost Impossible) | can't have couldn't have | He looks happy. He can't have failed his exam. |
💡 Pro Tip: Important Note on Negatives
To make a negative deduction (to say something was almost impossible), we use can't have or couldn't have. We do NOT use "mustn't have".
Incorrect: He mustn't have seen me.
Correct: He can't have seen me.
We use might not have to say something possibly didn't happen.
"He's not here. He might not have received my message."
Practice Your Grammar 🎯
Exercise 1: What Happened?
Choose the best modal form to complete each sentence.
- I can't find my wallet anywhere! I _______ it at the restaurant.
→ must have left - She didn't answer her phone. She _______ busy.
→ might have been / could have been - You say you saw him in Battambang this morning? You _______ seen him in Phnom Penh last night. It's too far to travel that fast.
→ can't have / couldn't have
Your Grammar Mission ⭐
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'.
- Evidence: The window is broken and a rock is on the floor inside.
(Example: Someone must have thrown a rock through the window.) - Evidence: Your friend looks very happy and excited today.
(Example: She might have received some good news.) - Evidence: The cake is all gone, but the plates are still on the table.
(Example: They can't have finished cleaning up yet.)
Key Vocabulary
- Past Deduction A logical guess about a past event based on present evidence.
- Evidence The available facts or information indicating whether a belief is true.
- Impossible Not able to occur, exist, or be done.
- To Speculate To form a guess about a subject without firm evidence.