Expressing Logical Deduction and Certainty: Must and Can’t
When we do not know a fact directly, but we use existing evidence or logical reasoning to draw a strong conclusion, we make a logical deduction. In English, we use must for positive deductions and can’t (cannot) for negative deductions.
1. Present Logical Deduction
To make a logical deduction about a situation in the present, we use the following structures:
A. Must (Positive Deduction: Almost 100% Sure Something is True)
- Structure:
Subject + must + Verb 1 / be + Adjective/Noun - Use: When all evidence points to one clear, positive conclusion.
- Example: She has been working for 12 hours. She must be exhausted. (Logic says she is tired).
- Example: The lights are on and we hear music inside. They must be home.
B. Can’t / Cannot (Negative Deduction: Almost 100% Sure Something is Impossible)
- Structure:
Subject + can't + Verb 1 / be + Adjective/Noun - Use: When we are sure something is impossible because it contradicts the evidence.
- Example: It is Sunday. The bank can’t be open today. (Logic says it is closed).
- Example: He just ate a huge lunch. He can’t be hungry already.
Note: We do not use must not for negative deduction; must not is reserved for prohibition. We use can’t for deductions.
2. Past Logical Deduction
To make a logical deduction about a past event based on current evidence, we use the perfect modal structure.
A. Must Have (Positive Past Deduction)
- Structure:
Subject + must + have + Verb 3 - Example: The ground is completely wet. It must have rained last night.
- Example: He won the first prize. He must have practiced very hard.
B. Can’t Have / Could Not Have (Negative Past Deduction)
- Structure:
Subject + can't/couldn't + have + Verb 3 - Example: I saw him in London an hour ago. He can’t have flown to New York yet. (It is physically impossible).
- Example: She didn’t answer her phone. She couldn’t have heard it ringing.
3. Summary of Deductions
| Evidence | Deduction Type | Modal Form | Example |
|---|---|---|---|
| Strong positive evidence | Present positive | must + be/Verb 1 | He must be rich. |
| Strong negative evidence | Present negative | can’t + be/Verb 1 | He can’t be poor. |
| Strong positive past evidence | Past positive | must + have + Verb 3 | They must have arrived. |
| Strong negative past evidence | Past negative | can’t + have + Verb 3 | They can’t have left. |
Next, we will explore Obligation & Duty to learn how to express commands, rules, and commitments using modal verbs.