B1 · Intermediate Chapter 31

Possibility and Probability

5 Total Rules
51 examples
6 min
Rules in This Chapter Quick Practice

Chapter in 30 Seconds

Master the art of expressing uncertainty and logical deductions with professional precision.

  • Differentiate between weak and strong possibilities using modal verbs.
  • Make logical deductions based on evidence using 'must' and 'can't'.
  • Use advanced probability adjectives and adverbs to sound more natural.
Stop guessing, start expressing levels of certainty.

What You'll Learn

Ever wonder how likely something is? This chapter helps you confidently discuss possibilities with may and might, and make logical deductions using must, so you can express yourself more precisely and naturally.

Learning Objectives

By the end of this chapter, you will be able to:

  1. 1
    By the end you will be able to: describe a 30-50% chance of an event happening using may or might.
  2. 2
    By the end you will be able to: conclude that something is definitely true or false based on evidence.
  3. 3
    By the end you will be able to: rank different outcomes from 'unlikely' to 'bound to' using non-modal structures.

Chapter Guide

Overview

As a B1 English learner, you're moving beyond basic communication and starting to express more nuanced ideas. This is where understanding English possibility and probability becomes incredibly useful! Imagine you're making plans, discussing news, or even just wondering about the weather – knowing how to express how likely something is can really boost your confidence and make your English sound much more natural. This chapter will equip you with essential tools to talk about uncertainty and make logical deductions.
We'll explore how to use modal verbs like may and might to express things that are simply possible, and how must helps you sound confident about a logical conclusion. You'll also learn handy phrases like may well and might well to express a higher degree of probability. By mastering these concepts, you'll be able to discuss potential outcomes, explain why you think something is true, and even confidently state when something just can't be! These are all key skills for navigating a wide range of everyday conversations at the B1 English grammar level.
Being able to differentiate between something that *might* happen and something that *must* be true based on evidence will help you communicate more precisely. It's about adding depth to your English, allowing you to share your thoughts and reasoning in a clear and convincing way. Get ready to discuss possibilities and probabilities like a pro!

How This Grammar Works

Let's break down the core grammar rules for expressing possibility and probability in English. We use modal verbs to do this, and they always precede a base verb (the infinitive without 'to').
When you're talking about something that is possible, but not certain, you'll use may or might. They are largely interchangeable for present or future possibility. For example, "It may rain later" means there's a chance of rain. Similarly, "She might be at the library" suggests it's a possibility, but you're not sure. These express general uncertainty or a 'maybe' situation.
Now, if you're quite sure about something based on evidence or logic, you use must. This isn't about obligation; it's about making a logical deduction. If your friend has been studying all night, you could say, "She must be tired." You're almost certain based on the clues. This is called the epistemic must. The evidence makes it highly probable.
To add a bit more strength to your predictions, you can use may well or might well. These phrases indicate that something is quite likely because it makes good sense or there are strong reasons for it. For instance, "If he doesn't study, he may well fail the exam" implies that it's a very probable outcome given his actions. It's stronger than just may or might alone, suggesting a reasonable expectation.
Finally, when you're absolutely certain that something is logically impossible, you use can't. For example, if you see someone enter a room and then immediately leave through the same door, you could say, "She can't be in the room anymore." The evidence tells you it's impossible. It's the opposite of must for logical deduction. These structures build on each other, allowing you to express varying degrees of certainty.

Common Mistakes

  1. 1✗ Using 'must' for permission instead of logical deduction in this context.
✓ He must be home, his car is in the driveway. (Correct: Logical deduction)
✗ You must go to the party. (Incorrect for probability; this is obligation. For possibility, you might say "You might go to the party if you want.")
*Explanation:* Remember that the 'must' we're focusing on here is for making a logical guess, not for giving orders or saying something is required.
  1. 1✗ Confusing 'may' and 'might' for past possibility.
✓ She may have forgotten her keys. (Correct for past possibility)
✗ She might forget her keys yesterday. (Incorrect: 'Might' for past possibility needs 'have' + past participle)
*Explanation:* For past possibility or probability, you need to use may have + past participle or might have + past participle. The chapter focuses on present/future, but this is a common extension.
  1. 1✗ Using 'cannot' instead of 'can't' for logical impossibility in informal contexts.
✓ That can't be true! (Correct: Common and natural)
✗ That cannot be true! (Less natural in everyday, conversational English for this specific meaning, though grammatically correct)
*Explanation:* While 'cannot' is grammatically fine, 'can't' is much more common and natural when expressing logical impossibility in conversational English.

Real Conversations

A

A

The lights are off in John's office. He usually works late.
B

B

He must have left already. It's almost 7 PM.
A

A

Do you think Sarah will come to the party?
B

B

She said she was busy, so she might not make it. But she may well surprise us if her meeting finishes early.
A

A

I can hear music from next door.
B

B

Oh, that can't be my neighbour. He's on holiday in Spain this week!

Quick FAQ

Q

Can I use 'may' and 'might' interchangeably for possibility?

Yes, in most cases for present and future possibility, may and might are interchangeable. Some people feel 'might' expresses a slightly weaker possibility, but the difference is very subtle and often not significant in everyday conversation.

Q

How is 'must' for possibility different from 'must' for obligation?

Must for possibility (epistemic) means "I am almost certain based on evidence" (e.g., "He must be rich; he drives a fancy car."). Must for obligation means "It is necessary or required" (e.g., "You must finish your homework."). The context makes the meaning clear.

Q

Is 'may well' much stronger than 'may'?

Yes, may well expresses a higher degree of probability than just may. It suggests that something is quite likely because there's a logical reason or expectation for it to happen. It's like saying "it's very likely" or "it's probable."

Q

When should I use 'can't' instead of 'won't be able to' for impossibility?

Use can't when you're making a logical deduction that something is impossible based on facts or evidence ("That can't be true, I saw it myself!"). Use 'won't be able to' for future inability or lack of opportunity ("I won't be able to come tomorrow, I have an appointment.").

Cultural Context

Native English speakers use these modal verbs constantly to soften statements, express uncertainty, and convey different levels of conviction. In casual conversation, you'll hear might and can't a lot, as they add a natural flow. Must for deduction is also very common. While may well and might well are perfectly natural, they tend to be slightly more formal or used when explaining a reasoned prediction. There aren't significant regional differences in the core usage of these modals, but the frequency of their use may vary slightly between individuals.

Key Examples (8)

1

I `may go` to the gym after work, if I'm not too tired.

English Possibility: May and Might
2

They `might be` stuck in traffic, which is why they're late.

English Possibility: May and Might
3

Look at all the snow! It `must be` freezing outside.

Logical Guessing with 'Must' (Epistemic)
4

He's wearing a doctor's coat. He `must be` a doctor.

Logical Guessing with 'Must' (Epistemic)
5

She **may well pass** the exam; she studied all week.

Probability Booster: May Well & Might Well
6

The package **might well arrive** tomorrow, as it shipped yesterday.

Probability Booster: May Well & Might Well
7

That pizza place `can't be closed` already, it's only 8 PM!

Logical Impossibility (It can't be!)
8

My internet `couldn't have gone out`. I just paid the bill yesterday.

Logical Impossibility (It can't be!)

Tips & Tricks (4)

💡

The 'Maybe' Test

If you can replace the word with 'Maybe' at the start of the sentence, you should probably use 'might'. Example: 'Maybe I'll go' -> 'I might go'.
frontend.learn_grammar.from_rule: English Possibility: May and Might
🎯

The 90% Rule

Only use 'must' if you are almost certain. If you are only 50% sure, use 'might' or 'could'.
frontend.learn_grammar.from_rule: Logical Guessing with 'Must' (Epistemic)
🎯

The 75% Rule

If you are more than 50% sure but less than 100% sure, 'may well' is your best friend. It sounds much more professional than 'maybe'.
frontend.learn_grammar.from_rule: Probability Booster: May Well & Might Well
💡

The Opposite Rule

Remember that 'can't' is the logical opposite of 'must'. If you are sure it's YES, use 'must'. If you are sure it's NO, use 'can't'.
frontend.learn_grammar.from_rule: Logical Impossibility (It can't be!)

Key Vocabulary (6)

Evidence facts or signs that show something is true Likelihood the chance that something will happen Certain completely sure about something Possible able to be done or happen Impossible not able to occur or exist Predict to say what will happen in the future

Real-World Preview

coffee

Solving an Office Mystery

Review Summary

  • Subject + may/might + base verb
  • Subject + must + base verb
  • Subject + can't + base verb

Common Mistakes

In English, we do not use 'must not' for logical deductions. We use 'can't' to say something is logically impossible.

Wrong: It must be not true.
Correct: It can't be true.

Modal verbs like 'might' never take an 's' in the third person singular.

Wrong: He mights come to the party.
Correct: He might come to the party.

Adverbs of probability like 'probably' usually go after the first auxiliary verb or before the main verb.

Wrong: Probably I will see you later.
Correct: I will probably see you later.

Next Steps

You've unlocked a new level of English! Being able to speculate and deduce makes your conversations much more interesting. Keep practicing those 'must be' moments!

Look out the window and make 5 'must be' and 5 'might be' observations about people you see.

Continue to: Collocations and Word Partnerships

Quick Practice (10)

Fix the error.

Find and fix the mistake:

He is bound that he wins.

✓ Correct! ✗ Not quite. Correct answer: He is bound to win.
Bound to + infinitive.

frontend.learn_grammar.from_rule: Probability: Likely, Unlikely, Bound To, Definitely, Probably

Correct the error in the sentence.

Find and fix the mistake:

She might well to be the winner.

✓ Correct! ✗ Not quite. Correct answer: She might well be the winner.
Remove 'to' after the modal verb.

frontend.learn_grammar.from_rule: Probability Booster: May Well & Might Well

Complete the past deduction.

You ___ (see) him yesterday; he was in Paris.

✓ Correct! ✗ Not quite. Correct answer: can't have seen
For past impossibility, use 'can't have' + past participle.

frontend.learn_grammar.from_rule: Logical Impossibility (It can't be!)

Choose the correct sentence.

Which is correct?

✓ Correct! ✗ Not quite. Correct answer: I will probably go.
Probably before verb.

frontend.learn_grammar.from_rule: Probability: Likely, Unlikely, Bound To, Definitely, Probably

Fix the mistake in the sentence.

Find and fix the mistake:

He mustn't be the thief because he has an alibi.

✓ Correct! ✗ Not quite. Correct answer: He can't be the thief...
Change 'mustn't' to 'can't' for logical deduction.

frontend.learn_grammar.from_rule: Logical Impossibility (It can't be!)

Which sentence expresses high probability?

Choose the correct sentence:

✓ Correct! ✗ Not quite. Correct answer: b
'May well' shows probability. 'May as well' is a suggestion. 'Well may' is wrong order. 'To go' is wrong after a modal.

frontend.learn_grammar.from_rule: Probability Booster: May Well & Might Well

Fill in the blank.

It is ___ to rain.

✓ Correct! ✗ Not quite. Correct answer: likely
Likely takes 'to'.

frontend.learn_grammar.from_rule: Probability: Likely, Unlikely, Bound To, Definitely, Probably

Fill in the blank with 'might' or 'may'.

It ___ rain later, so don't forget your coat.

✓ Correct! ✗ Not quite. Correct answer: might
We use the base form of the modal without 'to'.

frontend.learn_grammar.from_rule: English Possibility: May and Might

Choose the correct modal for logical deduction.

He has three Ferraris. He ___ be very rich.

✓ Correct! ✗ Not quite. Correct answer: must
Having three Ferraris is strong evidence of being rich.

frontend.learn_grammar.from_rule: Logical Guessing with 'Must' (Epistemic)

Fill in the blank with 'must be' or 'can't be'.

She's wearing a heavy coat and a scarf. It ___ very cold outside.

✓ Correct! ✗ Not quite. Correct answer: must be
A coat and scarf are evidence that it is cold.

frontend.learn_grammar.from_rule: Logical Guessing with 'Must' (Epistemic)

Score: /10

Common Questions (6)

In most cases, they are interchangeable. However, may is slightly more formal and suggests a higher probability (50%), while might is more common in speech and suggests a lower probability (30%).
Yes, but it is very rare, especially in American English. It's better to say might not.
Yes, but you need 'must have' + past participle. For example: 'He must have forgotten.'
Yes, 'must' is about 95% certain, while 'might' is only about 50% certain.
They are almost identical. 'May well' is slightly more formal and suggests a slightly higher probability than 'might well'.
No. In modern English, 'well' must come after the modal: Subject + may + well + verb.
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Collocations and Word Partnerships
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