Speculation and Deduction: sentences, exercises, tests and examples

Speculation and Deduction use

Speculation and Deduction is used when we speculate and make deductions based on what we know, see, or hear. With modal verbs, we can say that we are almost sure, that something is possible, that something is impossible, or that we expect something.

She looks exhausted. She must be working too hard.
He may have missed the train.
They can’t be at home. All the lights are off.
The parcel should arrive tomorrow.

Speculation and Deduction Form

In this topic, we often use different modal verbs depending on the degree of certainty and on whether we talk about the present, an ongoing process, or the past. For a completed past action, we usually use have + V3, and for a process happening around now, we use be + Ving.

must + V1 / be + Ving
may / might / could + V1 / be + Ving
can’t + V1 / be + Ving
must / may / might / could / can’t / couldn’t + have + V3
should / ought to + V1
should / ought to + have + V3

She must know the answer.
They might be waiting outside.
He can’t have forgotten your birthday.
The meeting should start soon.

Speculation and Deduction Rule

  • Must is used for a strong deduction: we are almost sure that something is true. It is not a 100% fact, but it is a very confident conclusion.
    She has been studying all day. She must be tired.
    Kate doesn’t have her car today. She must have rented one.
  • May, might, and could are used when something is possible, but we are not sure. Usually, may and might are very close in meaning, and could also often expresses possibility.
    The noise may be coming from the neighbours.
    He might have left already.
  • Can’t and couldn’t are used when we think something is impossible or when we show strong disbelief or surprise.
    You can’t be serious!
    Hannah couldn’t have said that. It doesn’t sound like her.
  • If we talk about something that is happening now as a process, we often use be + Ving after the modal verb.
    Amanda might be doing the washing up.
    He must be talking to the manager right now.
  • If the deduction refers to a completed action in the past, we usually use have + V3.
    ✅ She must have forgotten the keys.
    ✅ They might have taken a taxi.
    ✅ He can’t have seen the message.
  • Should and ought to in this topic often express an expectation: we think something is probably true or was expected to happen.
    The train should be here by now.
    They should have told him about the change.
  • For a weak negative possibility, we use may not and might not. The form couldn’t usually expresses impossibility, not just a weaker possibility.
    ✅ There might not be enough food for everyone.
    ❌ There couldn’t be enough food for everyone. (if you only mean “maybe not”)
  • Besides modal verbs, speculation can also be expressed with other words: be likely to, be unlikely to, be bound to, probably, and definitely.
    She is likely to forget about the meeting.
    That mistake was bound to happen sooner or later.

Speculation and Deduction Negation

In this topic, negative forms depend on the meaning. May not / might not = it is possible that something is not true. Can’t / couldn’t = it is impossible. Shouldn’t can express an expectation that something is probably not right.

may not / might not + V1
can’t + V1 / be + Ving
can’t / couldn’t + have + V3
shouldn’t + V1 / have + V3

He may not know the address.
They can’t be waiting outside in this weather.
She couldn’t have taken your bag by mistake.
The repair shouldn’t take much longer.

Speculation and Deduction Questions

In questions about speculation, we often use could, might, should, and structures like Do you think ...? Direct questions with must are possible, but they often sound more emotional or less neutral.

Could / Might + subject + V1 / be + Ving?
Wh-word + might / could + subject + have + V3?
Should + subject + V1?
Do you think + subject + might / could + V1?

Could she be sleeping?
What might have happened here?
Should they be here by now?
Do you think he could have missed the email?

Speculation and Deduction Common mistakes

❌ She must to be tired.
✅ She must be tired.
❌ He must has left.
✅ He must have left.
❌ They can’t have went home.
✅ They can’t have gone home.
❌ She mights be busy.
✅ She might be busy.
❌ He mustn’t have forgotten the meeting. (if you mean “that is impossible”)
✅ He can’t have forgotten the meeting.
❌ They should came by now.
✅ They should have come by now.

Speculation and Deduction Sentences

She isn’t answering. She might be having a meeting.
Look at those wet umbrellas. It must have rained.
This key can’t be the right one.
They may have taken the earlier bus.
The children are quiet. They must be sleeping.
He could be working from home today.
The guests should arrive around eight.
You can’t have seen Anna there. She is abroad.
There might not be enough chairs for everyone.
Do you think he might know the answer?

Speculation and Deduction Examples

The office is dark, so they must have left already.
Sarah might be waiting for us downstairs.
That sound could be coming from the kitchen.
He can’t be the new intern. He looks much older.
The flight should land in about twenty minutes.
She may not have understood what you meant.
They could have forgotten to lock the door.
It is unlikely to snow here in April.
This problem was bound to happen after the update.
Could he still be driving home?

English grammar exercises available in the app

Tenses

Future Simple Present Simple Present Perfect Present Continuous Past Simple Past Perfect Past Continuous Past Perfect Continuous Present Perfect Continuous Future Continuous Future Perfect Future Perfect Continuous Be going to

Conditionals

Second Conditional First Conditional Third Conditional Zero Conditional

Sentences

Relative Clauses Conjunctions Reported Speech There is / There are There was / There were

Verbs

Used to To be used to To get used to

Modals

Should or Shouldn't May or Might and Infinitive Modal Verbs Can / Can't Speculation and Deduction

Nouns and Articles

Countable and Uncountable Singular and Plural Nouns Uncountable and Plural