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What Do We Mean by Deduction?
'Deduction' is the act of forming an opinion or idea based on available information. These opinions are formed by finding a logical relation between the order of events.
Talking about Deductions in English
Here are some words that are used to make deductions in English:
Using 'Must'
You can use the modal verb 'must' when you are almost sure that something happens or is true based on observed evidence. You use 'must' with reference to the present or future tense. Check out the examples:
She looks wet, it must be raining.
Can you hear the loud music? There must be a party.
Using 'Have to'
'Have to' expresses the same meaning as the modal verb must. It has a similar function as well. You use 'have to' when you are almost sure that something is true.
She has to be a nurse, she is wearing nursing uniform.
You have to be Elina's mother, she really looks like you.
Using Could, Might, May
These modal verbs are used to talk about possibilities. This means we are making a guess based on what we know, but we are not sure about it.
She might give a mid-term exam today, the final exam is in a month.
Patrick may leave the counselor's room within a minute; the usual time for consultation is fifteen minutes.
'Cannot'
When it comes to deduction, the negative form of the modal verb 'can' (cannot) is exactly the opposite of the modal verb 'must.' It shows that something is impossible or very unlikely to be true, based on information or evidence. Keep in mind that in its affirmative form, 'can' does not express a deduction. Check out the examples.
He can't be john's father. He looks too young to be a father.
It cannot be her birthday, we went to her birthday party two months ago.
Deduction: Past Tense
There are different structures to refer to deductions about the past. Usually, we use these phrases to express our ideas about a past event based on available information or evidence.
Must have
Might have
May Have
Could have
Had to
Using 'Must Have'
We can use must + have before a past participle to indicate we are almost sure that something has happened in the past. Remember, in this case, we guess based on our personal knowledge or observations.
His eyes are red, he must have been crying.
Ian seems weird, she must have talked to him about the tapes.
Might Have, Could Have, May Have
The modal verbs 'may', 'might', and 'could' can be followed by the auxiliary have + past participles to offer deductions about the past with a lower degree of certainty.
Check out the examples for more clarifications.
Hanna might have been searching for you, she was at the library.
She could have started the fight, she is always aggressive.
Using 'Had to'
'Had to' is used with the same meaning as 'must have.' This means we can use 'had to' to talk about things that we are sure are true about the past. Check out the examples:
He had to be a part of this disaster.
That had to hurt when he fell.
Review
Some verbs are used to show that the speaker has reached a conclusion based on available information, evidence, or observations. These verbs include: 'must', 'have to', 'cannot', 'could', 'may', and 'might.' To make deductions about the past, you can add 'have' to the modals or use the past tense form of 'have to' which is 'had to.'
Quiz:
Which sentence expresses a deduction about the past with uncertainty?
He must have stolen the money.
He might have stolen the money.
He had to stolen the money.
He has to steal the money.
Sort the words to make a sentence.
Which sentence shows something is impossible based on evidence?
It must not be her birthday; we celebrated it last week.
It could be her birthday; she looks happy.
It can be her birthday; she likes parties.
It can't be her birthday; we celebrated it last week.
Match each sentence to option that best explains the kind of deduction it expresses.
Fill each blank with the correct modal/verb phrase. (There is one extra option.)
They
taken the wrong train; they arrived hours late.
This
be Michael's coat. It's way too small for him.
The puzzle
be easy — she solved it in under a minute.
They
be out of town. Their house has been dark for days.
The noise
be coming from the basement. I forgot to lock its door.
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