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What Do We Mean by Deduction?
'Deduction' is the act of forming an opinion or making a guess based on available information. These opinions are formed by finding a logical relation between the order of events.
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. You use 'must' with a reference to the present or future tense. Remember, after 'must' you have to use an infinitive without 'to.'
She looks wet, it must be raining.
Can you hear the loud music? There must be a party.
Using 'Have to'
The phrase 'have to' is used as the same meaning as the modal verb must. It has a similar function as well. You use 'have to' when you are almost 100% 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
The modal verbs are used to talk about possibilities. This means at the time we are talking about the very event, we are not certain whether the thing is true or not.
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.
Using 'Cannot'
When it comes to 'deduction,' the modal verb 'can' in the negative form ('cannot') is exactly the opposite of the modal verb 'must.' 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 on her birthday party two months ago.
Deduction: Past Tense
There are different structures to refer to the past tense. Usually, we use these phrases to express our opinion about a past event, that is formed by available information about the past.
Could have
Might have
May Have
Must have
Had to
Using 'Must Have'
We can use the phrase 'must have' before 'past participles' to indicate we are almost sure that something has happened in the past. Remember in this case, we guess based on our personal knowledge about the event.
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, could' can be added to the 'have + past participles' to talk about things that we are not certain about it.
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'
The phrase '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. Here is the key point: Use an infinitive without 'to' 'bare infinitive' after 'had to.'
Check out the examples to avoid confusion.
He had to be a part of this disaster.
Alexandra had to speak to her daughter, she has changed her manner toward us.
Review
Some verbs are used to talk about deduction. Here are the verbs: must, have to, cannot, could, may, might. To make past deductions you can use the verb phrase: could have, might have, may have, must have, 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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