Modal Verbs of Deduction: how to make assumptions in English

In life, it’s almost impossible to be 100% sure about anything. But luckily, English grammar lets us express different levels of possibility — whether we're talking about the present, future, or even the past.

So why are you reading this article? You must be interested in improving your English skills. Or maybe someone made you do it (but that can’t be true, right?). To express these kinds of assumptions in English, we use modals of deduction.

Contents

1. Modals of Deduction: meaning in context and usage examples
2. How to deduct about the present, past and future
3. Past modals of deduction
4. Continuous aspect
5. Practice exercise
6. FAQs

Modals of Deduction: meaning in context and usage example

When we’re not 100% sure about something but want to make a guess or draw a logical conclusion, we use modals of deduction. These modal verbs help convey the degree of certainty in our assumption. These constructions are commonly used in both spoken and written English, especially when analyzing situations, making conclusions, or forming assumptions.

Modals of deduction are modal verbs used to express assumptions or conclusions about something in the present, past, or future. They help convey different levels of certainty in our guesses.

These modals are used to express assumptions about present or past events. They help to show how certain we are about our assumption, based on facts and logic. For example:

• The lights are on. They must be at home.
• She might be busy now.
• He can’t be the culprit. He was with us all day.

Modal verbs can also be used to make polite suggestions or express cautious opinions, so that we don’t sound too direct or rude. For example:

• He might be in his office.
• You must be tired after such a long trip.
• She may not have received your message yet.

This makes communication more delicate and formal.

How to deduct about the present, past and future

In the present tense, we use “must”, “can't”, “may”, “might”, “could” followed by the bare infinitive (without “to”). They help us make assumptions about the current situation.

Although “might” and “could” are past forms, they are used to make assumptions about the present or future. The past forms make our statements more hypothetical and polite.

✓ Must — high certainty:

• He must be at work now.
• She must know the answer.
• They must be tired after such a long journey.

✓ Can't / Cannot — high certainty that something is impossible:

• She can't be at home.
• He can't be the thief.
• This story can't be true.

✓ Might / May / Could — possibility, but not certainty:

• He might be sleeping.
• She may be busy right now.
• They could be on their way.

Modal verbs “must”, “can't”, “might”, “may”, and “could” can also be used to make assumptions about the future. In this case, they are typically followed by the bare infinitive (the base form of the verb) or with the construction “be going to”.

✓ Must — strong certainty about the future:

• He must arrive on time if he leaves early.
• She must pass the exam with all that preparation.

✓ Can't — strong certainty that something is impossible in the future:

• He can't win the race; he's not fast enough.
• They can't be here by 5 p.m.; the journey takes longer.

✓ Might / May / Could — possibility in the future, but without certainty:

• She might come to the party later.
• It may rain tomorrow, so take an umbrella.
• They could be late if there’s traffic.

Past modals of deduction

In the past, we use “must have”, “can't have”, “might have”, “may have”, “could have” + Past Participle (Ved/3) to make assumptions or express thoughts about events that happened in the past.

These constructions help us draw conclusions based on available facts or evidence, but they also reflect a certain level of uncertainty. Thus, past modals of deduction allow us to make assumptions about past situations in a grammatically correct and precise way.

✓ Must have — strong certainty about the past:

• She must have left early.
• They must have forgotten about the meeting.

✓ Can't have / Couldn't have — strong certainty that something did not happen:

• He can't have forgotten my birthday.
• She couldn't have seen him yesterday.

✓ Might have / May have / Could have — assumption about possible situations in the past:

• They might have missed the train.
• He may have left his phone at home.

✓ Could have also sometimes conveys an unrealized possibility:

• I could have gone to the party, but I stayed home.

Continuous aspect 

We can also make assumptions about actions happening at the moment or actions that were ongoing in the past. Using modal verbs in the continuous aspect allows us to express a deeper level of speculation, emphasizing the ongoing nature of the activity. This can add nuance and make assumptions more precise and detailed.

In the present tense, we use the following constructions:

• must be + V-ing; 
• can’t be + V-ing;
• might be + V-ing. 

She must be working late. (high certainty that an action is ongoing)

Past modals of deduction have the following formulas:

• must have been + V-ing;
• can’t have been + V-ing;
• might have been + V-ing.

He might have been sleeping when you called. (assumption about an action that was happening at a specific moment in the past)

Now you know how to correctly use present and past modals of deduction. Practice and try to apply them in your own sentences.

You can learn more about modal verbs in the following articles:

• Modal Verbs of Willingness
• Modal Verbs of Necessity;
• Modal Verbs of Probability;
• Modal Verbs of Ability.

Practice exercise

Insert the correct modal verb. For self-checking, click on the sentence you need.

FAQs

1. Can we use “should” or “would” for deduction?

No, “should” and “would” do not express assumptions. They are used for giving advice and in conditional sentences.

“Should” usually means a recommendation or an expected result:

• You should see a doctor if you feel unwell.
• The train should arrive at 5 PM. 

“Would” is used in conditional sentences and polite requests:

• If I had more time, I would travel more.
• Would you like some coffee?

2. Is there a difference between “might have” and “could have”?

The meanings are similar, but there are nuances.

“Might have” means that something possibly happened:

• She might have forgotten about the meeting.

“Could have” can indicate both possibility and an unrealized action:

• He could have won the race, but he fell.
• I could have helped you, but you didn’t ask.

3. Can we say “must not have” for deduction?

No, “must” is only used in affirmative sentences for deduction. Therefore, the correct form to express negation is “can't have” or “couldn't have”.

• She must have left already.
• She can’t have left already.
• He couldn’t have seen me.

4. Can we use “may have” in formal writing? 

Yes, “may have” is acceptable in formal and academic writing:

• This may have contributed to the problem.
• The results may have been influenced by external factors.