Modals of Deduction

Welcome to today’s Lunchtime Lessons post. This week we are looking at modals of deduction and language of speculation. Our Lunchtime Lessons are free Online English classes where we look at areas of English which are often difficult for students.

If you didn’t manage to join us for our live session, you can catch up with the highlights of the class below. If you’d like to join our live session for the chance to ask the teacher questions, you can book your place directly on our website. These classes are 100% free and are on Tuesdays at 12pm

​Modals of Deduction
Welcome to today’s Lunchtime Lesson. Today’s lesson has a vocabulary focus with some grammar aspects. We’re going to be talking about modals of deduction.
You might remember, a few weeks ago, we did a Lunchtime Lesson on modal verbs of obligation. As a quick reminder, modal verbs are those special types of verbs which change or affect the main verb in a sentence. Modal verbs can have lots of different functions. They can be talk about possibility, ability, obligation or permission. As we have already discussed how we can use them to talk about obligation, today we are focusing on modal verbs we can use to talk about speculation or deduction. Do you already have an idea which modal verbs we use? Can you think of any modal verbs that express how sure we are that something is true or something is possible?
There are four main modal verbs we can us for speculation or deduction


  • must
  • might
  • may
  • can’t

The way we think of modals of deduction or speculation is different from how we think about the modals of obligation. When we looked at modals of obligation, we put them on a scale from 100% obligation – 0% obligation. When we’re thinking about speculation, it’s different. You can’t think of them as opposites. You have to think of how possible or impossible they are. So, you have to think of a scale going from 100% possible to 100% impossible, the intensity on either side of the scale is the same.

Common mistakes
Please remember and avoid the following common mistakes:


  • Pure modals such as must, might, may, can’t are followed by a bare infinitive
    e.g. it must to be true (wrong) it must be true (right)


  • Don’t use could when talking about speculation. Could is used to form a second conditional, which expresses something hypothetical e.g. It could be John, if he weren’t in Spain. If you want to say that it’s 50% possible that this is John you have to use might e.g. It might be John.


  • Don’t overuse may. Students often use may instead of might. May is generally used in a more formal context than might. The register of might is neutral to informal, which makes it the better option to use in your day to day conversations. Here are some examples for might and may from the Longman Dictionary. ​
Modals of Deduction can be used in a past aspect
A big difference between modals of speculation and modals of obligation is that we can use modals of speculation in a past aspect. You might remember, when we talked about modals of obligation, I told you that you can’t say for example, When I was at school, I must wear a uniform. We had to use have to, as we can put this into the past e.g. When I was at school, I had to wear a uniform. But our four main modals of speculation, must, might, may, can’t can be put in a past aspect. You must be careful though, must and can’t can also be used as a modal of obligation. So, it is very important that you analyze the sentence carefully to find out if the modal is expressing an obligation or  speculation. Remember: Context is key!


If I want to use one of the four main modal verbs in a past aspect, I have to follow this structure: must/might/may/can’t + have + participle
e.g. I must have left my keys at home.
When using those modals in a past aspect you need to think about your pronunciation.  The auxiliary verb have is not important. It’s not conveying meaning, which means when we’re speaking quickly and fluently, this verb contracts and we use the weak vowel sound, the schwa instead of the strong sound. By doing this, the three words must have been melt together to one block of sound. The same thing happens with any other of our four main modals. To practice and hear the difference between slow and quick pronunciation, start the video at 8.10 min.

Remember: If you can produce a sound, you can hear a sound. Meaning, using those contractions and working on your pronunciation is not only important for your own fluency, but will also help you understand proficient speaker better.
More Language for Speculation
Apart from those four modal verbs there is lots of other language we can use for speculation. Using other ways will make your English more interesting and quite often make you sound a bit more natural. If you’re doing any exams in English, speculation is one of the areas that candidates often forget to include in their language and it’s something that the examiners are always looking for. So, I’m going to show you some more vocabulary, which is brilliant for you in terms of your exam because it’s going to boost your marks. And also, if you just think about how you speak in general in your own language, speculation is such a big part of what we express.
First of all, let’s look at some adjectives we can use for speculation: bound to, sure to, likely and unlikely.​​
In case you don’t remember what, a clause is, a clause is a group of independent words that has meaning, but it doesn’t form a sentence. So, for example, the government will raise interest rates this year.
There are also some adverbs we can use for speculation: definitely and probably. These are two brilliant words to bring into your speaking and writing. Sadly, I hardly ever hear students using those words even though they aren’t complicated to use at all. The only thing you need to be careful about, is the position of those adverbs. The mistake people usually make is that they start a sentence with the adverb e.g. Probably, he won’t be here until 8.15. OR Definitely, she will pass the exam. But that’s wrong. Let’s have a look how to properly place adverbs of speculation.


  • Positive sentence: the adverb goes before the main verb and after the auxiliary verb (if there is one)
    She’ll definitely pass the exam.
    He’ll probably arrive around 8.15.
  • Negative sentence: the adverb goes before the auxiliary verb
    She definitely won’t pass the exam.
    He probably won’t arrive until 8.15.

If we have a sentence with the verb be, the following rules apply:

  • Positive sentence: the adverb goes after the verb be
    He’s probably British.
  • Negative sentence: the adverb goes before the verb be
    The painting definitely isn’t genuine.

A little side not: be likely to is very similar to probably in meaning. However, the register is different. Be likely to is more formal. It is a great expression to bring into your written English. Probably is less formal and therefore perfect to bring into your spoken English.
As I mentioned earlier, in terms of exams, examiners are looking for that language of speculation. And in pretty much each part of your speaking exam, you can use language of speculation. When you have to describe photos in part 2 of the Cambridge exams , the task always asks what might the people be doing or how might the people be feeling. Which means you have to speculate. But also, in part three and four you can bring in language of speculation as it just makes your language so much richer and it shows that you’re able to analyse a topic.
In terms of writing, language of speculation can be used in many areas. It works fantastically in proposals, reports, and essays but also in an informal writing.
So, my challenge for you this week is to try and bring in language of speculation as much as possible into any English that you’re producing. If you’re going to make a sentence, think about whether there is space for you to use some of the language that we’ve talked about today. Push yourself, challenge yourself! If you’re already comfortable with using those modal verbs, try some of the other expressions we looked at today. If you like, leave us a comment and speculate about something.

Scroll to Top