Speculation and deduction

  • B2+-C1
  • Grammar
  • Modals and related verbs

This article focuses on using modal verbs and adjectives for certainty, probability, and deduction. “Deduction” means using the information available to make a guess or draw a conclusion about the facts, and depending on the information available, you might be more certain that your conclusion is true, or less certain that your conclusion is true – and we use different modal verbs or adjectives to indicate the degree of certainty.

modal verbs: must / may / might / can’t / should
  1. Mel and Trudy must be very well off – they’ve got an enormous house.
  2. You must have seen him – he was standing right in front of you!
  1. They can’t be playing very well – they’re losing 0-3.
  2. You can’t / couldn’t have spent very long on this essay – you’ve only written 100 words.
  1. I haven’t seen the sales manager today. He may / might / could be off sick.
  2. The keys of the store cupboard have disappeared. Do you think someone may / might / could have taken them?
  3. He may / might not have heard the message I left on his voicemail.
  1. If I post the letter today, it should arrive on Friday.
  2. I posted the letter a week ago. It should have arrived by now.
  1. As well as for obligation, we also use must + infinitive to say that we are almost sure something is true about the present, and must have + past participle to say that we are almost sure something was true or happened in the past.
  1. We use can’t and can’t / couldn’t + perfect infinitive (NOT mustn’t / mustn’t have) to say that we are almost sure that something isn’t true in the present or didn’t happen / wasn’t true in the past.
  1. We use may / might / could and may / might / could + perfect infinitive to say that we think it’s possible that something is true in the present or was true / happened in the past.
  2. Compare:
  3. He might not have done it. (= Maybe he didn’t do it.)
  4. He couldn’t have done it. (= It is impossible that he did it.)
  1. Use should + infinitive (or should have + participle) to describe a situation you expect to happen (or would expect to have happened in the past).
  • Grammar checkpoint
    ⚠ Compare the use of the infinitive and the continuous infinitive after these modals:

    • He must work really hard. He never gets home before 9.00 p.m. = deduction about a habitual action
    • There’s a light on in his office. He must still be working. = deduction about an action in progress at the moment of speaking
adjectives and adverbs for speculation
  1. He’s bound / sure to be here in a minute. He left an hour ago.
  2. She‘s sure to know. She’s an expert on the subject.
  1. I think she‘s likely to agree to our proposal – we’ve given her some very good reasons.
  2. The doctors say that at his age he‘s unlikely to recover.
  3. I think that it’s very likely that the meeting will be over by 6.00.
  4. It’s unlikely that the government will raise interest rates this year.
  1. She‘ll definitely pass the exam. She’s worked really hard.
  2. She definitely won’t pass the exam. She hasn’t done any work at all.
  3. He‘ll probably be here around 8.00. He usually leaves work at 7.30.
  4. He probably won’t be here until about 8.15. He’s stuck in a traffic jam.
  1. Bound and sure are adjectives. We use be bound or be sure + infinitive to say that you think something is certain to be true or to happen.
  1. Likely / unlikely are also adjectives (not adverbs). We can use subject + be likely / unlikely + infinitive, or it is likely / unlikely + that + clause.
  1. Definitely and probably are adverbs. They go before a main verb and after the auxiliary (if there is one) in positive sentences and before the auxiliary verb negative sentences.
  2. With be they go after the verb in positive sentences and before it in negative sentences, e.g. He’s probably British. The painting definitely isn’t genuine.
  • Grammar checkpoint
    ⚠ be likely to and will probably are very similar in meaning, but be likely to is more formal. Compare:

    • The new CEO is likely to be appointed today.
    • The new CEO will probably be appointed today.
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Nigel Bailey

Nigel is a qualified Teacher of English as a Foreign Language and has been teaching in Poland since 2003. He has been teaching over the Internet since 2008 and has taught more than 1,000 students in this way.

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