IELTS Grammar: Modal Verbs For Deduction

English language grammar for IELTS offers a wide range of topics when it comes to perfection in the language. Here we come up with our new topic of grammar, that is, modal verbs for deduction.

Modal verbs for deduction are used:-

● To guess if something is true using the available information.

● To show how certain we are about the possibility or to check the level of certainty.

Today’s article will describe the detailed use of modal verbs for the certainty of possibility, probability, and deduction.


“Deduction” means to use the information available to make a guess or to draw a conclusion about the facts available. Different modal verbs are used to indicate the degree of certainty. Some modal verbs of deduction are 'must', 'should', 'could', 'will', 'can', 'have to', 'might', 'may'.

How certain are you?

Use these modal verbs:

100% – Completely sure almost certain

must, can’t, couldn’t

80% – Expecting to be certain


50% – Maybe certain

might, may, could

Modals For Deduction About Present

To make deductions about the present,
● Use must if one is sure about something to be true.
● Use can’t if one is sure that something is impossible.

For example

Carla works every day from 9 AM to 5 PM. Right now it’s 10 AM, so…

▪ She must be at work right now.

▪ She can’t be at home.

Modals For Deduction About Past

Whenever we are available with a present situation or evidence and we tend to draw a conclusion about what was happened in the past, the modal verbs for deduction which are used

● Must have

● Couldn't have

Along with a past form of verb.

Must Have + Past Participle

  1. Sheila got a tan. She must have spent a lot of time in the sun lately.
  2. There was one banana left, but now it’s gone. My husband must have eaten it.

Must Not Have + Past Participle

  1. The car is still dirty. Paul must not have washed it yet.
  2. He barely touched his lunch. He must not have been hungry.

Couldn’t Have + Past Participlewhen one is certain that some situation was impossible to happen in the past.

  1. Martha couldn’t have taken your notebook; she wasn’t even in class yesterday.
  2. The cookies are gone. But Eric couldn’t have reached the cookies on the top shelf; he must have asked his older brother to get them.

Let's look for an example that represents the difference between must not have and couldn’t have:

▪ I did not see the report here – she must not have printed it out. (Printed- verb's past form)
(we draw the conclusion that she did not do it)

▪ The printer has been broken for the past week, so she couldn’t have printed out the report. (Printed- verb's past form)
(we know it was IMPOSSIBLE for her to do it)

Modals For Deduction About The Future

Modals for deductions about the future if we are making predictions. As you know about the future tenses, one can use will or going to for saying what one believes will happen in the future. When one is quite sure that the made prediction is certain, the word 'definitely' can be added to put focus on the prediction.

1. She is definitely going to love this book – it’s by her favorite author.

2. The kids will definitely be thrilled when we tell them we’re going to Essel World.

Two slightly more formal expressions that can be used for talking about the future with certainty is saying that something is certain to happen, or is sure to happen:

▪ Engineers are certain to develop even faster computers.

▪ The country is sure to come to the aid of its ally.


Below is a guide to all the modals in detail. Their usage, description and examples have been illustrated individually below.

▶️ Must

Modal verb must is used when one feels certain that something is true or it's the only realistic possibility.

● This must be her house. I can see her car in the garage.

● He must live near here because he always walks to work.

● You have worked all day, you must be tired.

● He must live near here because he comes to work on foot.

● She’s not here. She must be in the kitchen.

● Come inside and get warm. You must be freezing out there!

● You are a pilot, that must be interesting.

● He must be around here because his car is parked there.

● He has an expensive car, he must have some money.

● She isn’t answering the phone, she must be out.

● He drives an expensive car, he must have a good job.

● You’re a zookeeper? That must be very interesting.

● Her son is at university so she must be at least 40.

● Carla works every day from 9 AM to 5 PM so she must be at work.

▶️ Might, May, Could

The modal verbs might, may or could are used to say that something is possible but not so certain.

● She's not here yet. She might be stuck in traffic.

● We regret to inform you that some services may be delayed due to the bad weather.

● I heard a noise, there might be a cat on the roof

● She might be going to Australia.

● They might be some kind of small pig

● He might be in the Gym, sometimes he goes there

● Jerry is very tall so he might be good at basketball.

● She is busy, she might not be able to come tonight.

● He might be in his bedroom.

● He's not answering. He could be in class.

● Peter could be at the library.

● Don’t eat it! It could be poisoned!

● Don’t put it up there. It could fall off and hit someone.

▶️ Can't

The modal verb can't is used when one feels that something is not possible.

● It can't be easy for him, looking after three kids on his own.

● He can’t be dead, I just talked to him on the phone

● My wallet can’t be in my backpack, I already checked there

● It can’t be a mechanical problem, the car is new

● It can't be far now. We've been driving for hours.

● That can’t be James, James wears glasses

● That looks like tomato juice, but it can’t be, that would be too easy.

● She can’t be a mother, she’s only 14!

● He can’t be in prison, I saw him yesterday in a pub.

● They can’t be Spanish, they’re speaking Portuguese

▶️ Must Have

The modal verb must have is used when one feels sure about what happened. It is used along with the past participle form of the verb.

● The thief must have had a key. The door was locked and nothing was broken.

● Who told the newspapers about the prime minister's plans? It must have been someone close to him.

● Oh, good! We've got milk. Mo must have bought some yesterday.

▶️ Might Have / May Have

The modal verb might have or may have is used when we think it's possible that something happened. It is used along with the past participle form of the verb.

● I think I might have left the air conditioning on. Please can you check?

● Police think the suspect may have left the country using a fake passport.

● She might have gone to the shops."

May have is more formal than might have. Could have is also possible in this context but less common.

▶️ Might Not Have

The modal verb might not have is a negative form of might have modal verb. It is used in a sentence where it's possible that the action did not happen, but not certain.

● "He might not have finished his exams yet."

It is possible that he has not finished his exams, but it is also possible that he has finished them.

▶️ Can't Have / Couldn't Have

The modal verbs can't have and couldn't have are the negative forms of can have and could have. These are used when we think it's not possible that something happened in the past. The past participle form of the verb is used along with these modal verbs.

● She can't have driven there. Her car keys are still here.

● I thought I saw Adnan this morning but it couldn't have been him – he's in Greece this week.

● She can’t leave the house yet because her car is still outside."

So above-mentioned information about the detailed usage of modal verbs for deduction will help you in finding its correct place in a sentence. Practice for this grammar topic and keep in touch for more of the same.

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