Deduction คือการใช้ modal verbs ในประโยคที่ผู้พูด ‘ไม่ได้มั่นใจ’ ว่าสิ่งที่ตัวเองพูดนั้นจะเกิดขึ้นจริง ยกตัวอย่างเช่น ไม่แน่ใจว่าวันนี้เพื่อนอยู่ที่ไหน คุณอาจพูดได้ว่า
She must be at work.
She might be having a meeting.
She could be working on a project.
She can’t be taking the day off.
ซึ่งในแต่ละประโยค ผู้พูดคาดหวังว่าสิ่งที่พูดนั้นจะเป็นจริง แต่ก็ไม่ได้มั่นใจ 100% นั่นเองครับ เพราะถ้าหากว่าผู้พูดมั่นใจในสิ่งที่พูด ก็จะสามารถใช้ present หรือ present continuous ได้ เช่น
She’s at work.
She’s having a meeting.
She’s working on a project today.
She isn’t taking the day off.
ทีนี้เราลองไปดูกันดีกว่าครับว่า การใช้ Modal verbs of deduction ในแต่ละแบบจะเป็นอย่างไรบ้าง ทั้งใน present และ past เลยครับ
Present Modal Verbs of Deduction
must be / do / go / etc. (I’m almost 100% certain that it’s true.)
He must be at the bank. – He told me earlier, so I’m fairly certain he is there.
They must do that type of work. – It makes good sense that they would do that type of work, but I’m 100% sure.
Susan must go to an expensive hairdresser. – I’m 90% sure because her hair is very nice, but I don’t know for certain.
might / may be / do / go / etc. (It’s a good possibility.)
Peter might come to dinner tonight. – I’m not sure, but I know he would like to come.
They may want to have dinner when they get here. – I know they are traveling, so I think it’s a good possibility.
She might be a good choice for the job. – I think she’s got the right qualifications, but I don’t know who they will hire.
could be / do / go / etc. (It’s one of many possibilities.)
We could go to dinner now. – That’s my idea, it’s possible, but I don’t know if others want to do it.
They could be doing homework now. – It’s a possibility, but I know they like to do many other things.
She could be at school. – Or she could be in a restaurant, or maybe at work, there are many possibilities.
can’t / couldn’t be / do / go / etc. (I’m almost certain that something isn’t true.)
She can’t be cooking chicken. – I know she hates chicken.
They couldn’t be doing their homework. – It’s a Saturday night. They never do homework then.
Alan can’t have time off. – He rarely gets time off, and he just went on a vacation last month.
Past Modal Verbs of Deduction
must have done / been / gone / etc. (I’m almost 100% certain that it explains what happened in the past.)
She must have done her homework. She never forgets. – I’m sure she did her homework because she always does it.
Peter must have gone to a doctor if he felt bad. He always goes to the doctor when he feels bad. – I know Peter well, so I know he probably went to see a doctor.
Sharon must have been hurt by her boyfriend. – I know they broke up, so I’m sure she felt badly when it happened.
might have been / done / gone / etc. (There’s a good possibility that it explains what happened in the past.)
They might have gone out for dinner. – They’re not here and that’s a good possibility.
She might have been ill. – I think that’s a good guess because I know her health isn’t the best.
Andrew might have done the yard work. – I know Andrew is one of the people who usually do the yard work.
could have done / been / gone / etc. (It’s one explanation for what happened in the past)
They could have been lost. – It’s a possible explanation why they had problems.
Mark could have done the work. – Mark’s one of the people who do this type of work.
Jennifer could have gone. – Going home was one of the possibilities
can’t (UK English) / couldn’t (US English) have been / done / gone / etc. (I’m sure that it’s not an explanation for what happened in the past. )
They can’t have bought that car. – I know that they didn’t want to buy that car.
Mary couldn’t have finished the report. – I know she didn’t have enough time to finish the report.
Jason can’t have forgotten the appointment. – I know that Jason never forgets appointments.
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