To express certainty in the present, we use must, can’tandcouldn’t.
To express certainty in the past, we use must have, can’t have and couldn’t have.
- a)I can’t find my keys. They must be here somewhere because I used them to open the front door.
- b)Theycan’t be outside, because I came inside immediately.
- c)I must have put them back in my handbag. Oh yes here they are!
To talk about probability in the present or future, we use should and ought to.
- d)Youshould bebetter soon if you take this medicine.
- e)You ought to recover quickly if you follow the doctor’s orders.
To talk about it in the past, we use should have and ought to have.
- f)My rash should have gone by now. I have been using this cream exactly as the doctor told me to.
- g)I ought to have had a lower fever by now. I have been taking medication.
We use could, may and might to talk about the possibility of something either in the present or in the future.
- h)I think you should go back to the doctor. That rash could/may/might get worse, and you might have scars later.
We use could have and might have + past participle to talk about possibility in the past.
- i)I could/might have died if I hadn’t gone to hospital when I did.
Exercise 1.- Grammar
Complete each sentence with one word.
- a)They aren’t answering their phones. They ____________ be somewhere without a signal.
- b)Elvis Presley_______________ be alive. Someone would have seen him by now.
- c)The passengers on the flight _______________ if the pilot hadn’t reacted so quickly.
- d)I don’t know why I’m still feeling ill. I took the medicine, so I _______________ feeling better by now.
- e)She_______________ her keys in the car.
- f)You _______________ He is on holiday in Australia.
- g)The students _______________very nervous, which is why they didn’t do well in the exam.
- h)She doesn’t know when she got home. She _______________so tired that she can’t remember.