English — Expressing Certainty and Uncertainty (Must, Can’t, May, Might, Could) — Grammar Rules, Examples and Exercises — 4th Year Middle School — Algerian Curriculum

Lesson: Expressing Certainty and Uncertainty — Must, Can’t, May, Might, Could

Learning Objectives:

• Understand how to express certainty and uncertainty in English.
• Use “must” and “can’t” for strong certainty (positive and negative).
• Use “may”, “might”, and “could” for possibility/uncertainty.
• Practice making deductions and guesses in different contexts.

I. Introduction

When we talk about how sure we are about something, we use modal verbs of deduction and possibility. These modals help us express how certain or uncertain we feel.

II. Expressing Strong Certainty

1. “Must” — Very certain (positive)

We use must when we are almost 100% sure that something is true based on evidence.

• He has been working all day. He must be tired.
• The light is on. She must be at home.
• They have a big house. They must have a lot of money.

2. “Can’t” / “Cannot” — Very certain (negative)

We use can’t when we are almost 100% sure that something is NOT true.

• He just ate lunch. He can’t be hungry.
• She is only 10 years old. She can’t be a doctor.
• It’s midnight. He can’t be at the office now.

III. Expressing Possibility/Uncertainty

1. “May” — Possible (50% certainty)

• It may rain later, so take an umbrella.
• She may come to the party tonight.
• The meeting may be cancelled.

2. “Might” — Less possible (around 30% certainty)

• I might go to the cinema this evening.
• They might arrive late because of traffic.
• He might not know the answer.

3. “Could” — Possible (similar to might)

• That could be true, but I’m not sure.
• She could be at the library.
• This could be the solution to the problem.

IV. Comparison Table

Modal Verb	Level of Certainty	Meaning
Must	95-100%	Almost certain (positive deduction)
Can’t	95-100%	Almost certain it’s not true
May	Around 50%	Possible, perhaps
Might	Around 30%	Less possible
Could	Around 30-50%	Possible, one option

MUST (100%) → MAY (50%) → MIGHT/COULD (30%) → CAN’T (0%)

V. Examples in Context

Example 1 — At home:

A: Someone is knocking at the door. Who is it?

B: It might be the postman. Or it could be our neighbour.

A: I hear a car. It must be Dad returning from work.

Example 2 — At school:

A: Where is Ahmed? He is not in class today.

B: He might be sick. He looked tired yesterday.

C: No, I saw him this morning. He can’t be sick. He must be with the headmaster.

VI. Exercises

Exercise 1: Fill in the blanks with “must”, “can’t”, “may”, “might”, or “could”.

1. He has a car, a big house and a boat. He ______ be rich.
2. The baby is sleeping. You ______ wake her up.
3. I’m not sure, but I ______ visit my grandmother tomorrow.
4. She is very good at English. She ______ be from England.
5. Where is my phone? It ______ be in the kitchen or in the bedroom.

Exercise 2: Rewrite the sentences using the correct modal verb.

1. I’m sure it’s not true. → It ______ be true.
2. Perhaps she is at home. → She ______ be at home.
3. I’m almost certain he is the best player. → He ______ be the best player.

VII. Answer Key

Exercise 1:

1. must (strong evidence)
2. can’t (it’s not allowed/possible)
3. may/might/could (uncertainty)
4. must (strong deduction)
5. may/might/could (possibility)

Exercise 2:

📍 دروس مشابهة

• Passive Voice (Active to Passive) in Different Tenses — Grammar, Rules, Examples
• Gerunds and Infinitives (Verb + -ing vs Verb + To Infinitive) — Grammar Rules, E
• First Conditional (If Clause Type 1) — If + Present Simple + Will — Grammar Rule

1. can’t be true
2. may/might/could be at home
3. must be the best player

Summary:

• Use must when you are almost certain something is true.
• Use can’t when you are almost certain something is false.
• Use may, might, or could when you are not sure (possibility).
• The choice depends on how much evidence you have.

Similar lessons:

• Passive Voice — English 4th Year Middle School
• Gerunds and Infinitives — English 4th Year Middle School