Modal verbs of deduction: Must, might, could, can’t — Explanation

This B1 exercise test will help you practice modal verbs of deduction. You can learn must, might, could, and can't by filling in the blanks with clear explanations.

Modal Verbs of Deduction: Must, Might, Could, Can’t Exercises

In English, we often use modal verbs of deduction to make guesses or come to logical conclusions about a situation. These modal verbs help us show how sure we are about something based on evidence or facts.

Must, might, could, and can't are the most common modal verbs of deduction at the B1 level.

Using “Must” for Strong Certainty

We use "must" when we are almost sure that something is true. The proof is strong, and the conclusion makes a lot of sense.

Structure:
must + base verb

Examples:

• She studies every day and gets good grades. She must be very hardworking.
• The lights are on and the car is outside. They must be at home.
• He looks very tired. He must need some rest.

Explanation:
Use must when you are very sure of your deduction.

Using "might" to show that something is possible

When we say "might," we mean that something is possible but we don't know for sure.

Structure:
might + base verb

Examples:

• She is not answering her phone. She might be busy.
• It’s cloudy today. It might rain later.
• He isn’t in the office. He might work from home today.

Explanation:
Use might when you think something might happen but don't have strong proof.

Using “Could” for Possibility

We use could in the same way as might. It also shows what could happen, and it is often used in place of "might."

Structure:
could + base verb

Examples:

• I can’t find my keys. They could be in my bag.
• She isn’t here yet. She could arrive soon.
• This restaurant is empty. It could be closed.

Explanation:
Use "could" when something is possible. It is a little more neutral and open than might.

Using “Can’t” for Strong Negative Certainty

When we are sure that something is NOT true, we say "can't."

Structure:
can’t + base verb

Examples:

• He is only five years old. He can’t drive a car.
• She just left the office. She can’t be at home already.
• That answer is wrong. It can’t be correct.

Explanation:
When you are sure that something is impossible, use can't.

Summary Table

Modal Verb	Meaning	Certainty Level	Example
must	strong certainty (positive)	very sure	She must be tired.
might	possibility	not sure	She might be tired.
could	possibility	not sure	She could be tired.
can’t	strong certainty (negative)	very sure	She can’t be tired.

Important Grammar Rule

You should always use the base form of the verb after modal verbs like must, might, could, and can't.

Correct:
She must be at home.

Incorrect:
She must is at home.