Mastering Modals: Expressing Possibility, Probability, and Certainty (C1 English Grammar) — C1 English grammar lesson by Langcom

Mastering Modals: Expressing Possibility, Probability, and Certainty (C1 English Grammar)

ラウル・マグイニャ

Langcom 創設者 | ケンブリッジおよびSIELE試験の試験官

9月 26, 2026

C1✏️ 文法⏱️ 7 min read📅 2026年9月更新
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To express possibility, probability, or certainty in C1 English, you use a range of modal verbs and specific phrases that show how likely something is to be true or to happen. For general possibilities, we often use can. For specific events, may, might, 、および could are common. You can make these possibilities stronger or weaker by adding adverbs like well, easily, possibly, 、または conceivably. For higher probability, phrases like is likely to are useful, while must indicates certainty based on evidence, and can’t expresses certainty that something is impossible.

What Are Modal Verbs for Possibility and Probability?

Modal verbs help us talk about how certain we are about something. They show different levels of possibility, from very likely to very unlikely.

The main modal verbs for expressing possibility about specific events are may, might, 、および could.

  • 当社では may (not), might (not), 、または could (but not could not for possibility) when we think something is possible, but we are not sure.
  • For example: The train may be delayed due to the bad weather. または She might not come to the party tonight.

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How Do We Express Different Degrees of Possibility?

You can make your statements about possibility stronger or weaker by adding specific adverbs or using different phrases.

Strong Possibility

  • 使用方法 may well, might well, 、または could easily to show a strong possibility.
  • 例: If you don't study, you could easily fail the exam.
  • 例: The new policy may well lead to more job opportunities.

Remote Possibility

  • To show a very small or remote possibility, use may possibly, might conceivably, 、または just might.
  • 例: I just might have time to help you tomorrow, but it's unlikely.
  • 例: My boss might conceivably agree to the proposal, but I wouldn't count on it.

When Do We Use 'Can' for Possibility?

The modal verb can is also used for possibility, but in a different way than may, might, 、または could.

  • 当社では can to talk about general possibilities or things that are sometimes true. It refers to what is generally possible, not a specific event.
  • 例: It can get very cold in winter here. (This means it's a general characteristic of winter in this place.)
  • Compare this to: It might get cold tonight. (This refers to a specific possibility for tonight.)
  • また、当社は can't to express certainty that something is impossible.
  • 例: That can't be true!

What Other Phrases Express Possibility and Probability?

Beyond modal verbs, there are many useful phrases to express different levels of possibility and probability.

For Possibility

  • 使用方法 It's (just/quite/very/entirely) possible that + clause.
  • 例: It's entirely possible that we will finish the project ahead of schedule.
  • 使用方法 There's (a/some/a slight/every/a good/strong/real) possibility/chance that + clause または of + -ing verb.
  • 例: There's a good chance that she'll get the promotion.
  • 例: There's a slight possibility of rain later.

For Probability

  • 使用方法 is likely/unlikely to + infinitive または It's likely/unlikely that + clause.
  • 例: He is unlikely to accept the offer.
  • 例: It's likely that the meeting will be postponed.
  • You can intensify these with highly または very: It's highly unlikely that they will agree.

How Do We Express Certainty and Impossibility?

When you are very sure about something, or sure that something is not true, you use different modals.

For Certainty (Deduction)

  • 使用方法 must when you are almost certain something is true based on evidence.
  • 例: She's been working all day; she must be exhausted.
  • 使用方法 can't または couldn't when you are certain something is impossible.
  • 例: He can't be serious!
  • 例: That couldn't be true.
Summary of Expressions for Possibility, Probability, and Certainty
Degree Modal Verbs / Phrases 例
General Possibility can Accidents can happen.
Remote Possibility might conceivably, just might, It's just possible that She just might agree.
Possibility may, might, could, It's possible that He may be late.
Strong Possibility may well, might well, could easily, There's a good chance that They may well win.
High Probability is likely to, It's highly probable that She's likely to succeed.
Certainty (Deduction) must You must be tired.
Certainty (Impossibility) can't, couldn't That can't be right.

例

  • The photocopier isn't working – there may be some paper stuck inside.
  • It can rain heavily in this region in autumn.
  • But it might rain this evening.
  • Children can be very irritating.
  • But you may find my children annoying when they make a lot of noise.
  • The weather may well improve by the weekend.
  • I'd better write it down; otherwise, I could easily forget.
  • My boss could conceivably change her mind and decide to give me a pay increase.
  • I just might have time to finish that report this week.
  • It's just possible that we'll finish the project by March.
  • There's every possibility that the business will expand next year.
  • If you've had no joy applying for positions this way, it may well be more productive to start hunting for less visible vacancies instead.
  • If you're answering ads for junior jobs in media, applying blind is highly unlikely to reap rewards.
  • In the long run you might easily conclude that the most treasured aspect of your university experience wasn't your academic education or any careers advice, but rather the friends you made.

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Spanish speakers often make specific mistakes when expressing possibility and certainty in English, mainly due to direct translation from 'poder'.

  • Confusing can そして may/might: In Spanish, 'poder' can mean both general ability/possibility and specific possibility. In English, can is for general possibility or ability, while may/might/could are for specific possibilities.
    • 誤り: It can rain tomorrow. (Unless you mean it's generally possible for it to rain, not a specific forecast.)
    • 正しい: It might rain tomorrow. (For a specific event.)
  • 使用方法 could not for possibility: Could not usually means impossibility or past inability, not simply 'not possible'.
    • 誤り: She could not be at home now. (If you mean 'it's possible she's not at home.')
    • 正しい: She may not be at home now. または She might not be at home now.
    • Correct (for impossibility): She couldn't be at home; I just saw her at the office.
  • 使いすぎ maybe: 一方で maybe is correct, C1 learners should use a wider range of expressions like perhaps, it's possible that, or modal verbs with adverbs to sound more natural and sophisticated.

よくある質問

What is the main difference between 'may' and 'might' for possibility?

両方とも may そして might express possibility, and often they are interchangeable. However, might can sometimes suggest a slightly lower or more remote possibility than may. 例えば、, It may rain suggests a reasonable chance, while It might rain could imply a slightly less certain chance.

Can I use 'couldn't' to express possibility?

No, couldn't is typically used to express certainty that something is impossible or to talk about a past inability. To express that something is possibly not true, use may not または might not. 例えば、, He couldn't be the thief means 'it's impossible he's the thief', whereas He may not be the thief means 'it's possible he's not the thief'.

How can I make my predictions sound more certain in English?

To make predictions sound more certain, you can use phrases like will definitely, is bound to, is sure to, 、または must (for deductions based on strong evidence). For example, instead of It might be difficult, you could say It's bound to be difficult if you are very certain.

How are these modals tested in the C1 Advanced (CAE) exam?

In the C1 Advanced (CAE) exam, expressing possibility, probability, and certainty is often tested in the Use of English section, particularly in Part 4 (Key Word Transformations) where you might need to rephrase sentences using different modals or probability phrases. It's also crucial for your Writing and Speaking papers to demonstrate a sophisticated range of expressions for nuanced communication.

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著者について:ラウル・マグイニャ

ラウル・マグイニャ氏は、認定Cambridge English試験対策センターである「Asociación Educativa Langcom」の所長であり、 あなたの国. 。語学教育および国際資格認定の分野で10年以上の経験を持ち、25カ国にわたる数千人の学生、教師、教育機関が、ケンブリッジ英語検定、IELTS、TOEFL、およびMichiganの英語資格を取得できるよう支援してきました。.

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