Category: General English & Academic English
Expressing Past Ability:
To express past ability, could, was/were able to should be used. For past ability, was or were able to is more common than could.
Present: I can edit and revise my assignment myself.
Past: I could edit and revise my assignment myself.
Present: I am able to fix the burnt spaghetti.
Past: I was able to fix the burnt spaghetti.
Degree of certainty in present time:
When we want to express how much we are sure about something to happen, we use modals to show degree of certainty.
100 % Sure:
When we are completely sure about an action, we necessarily don’t use any modals. For example, if I say, “Stephen is on a trip to Canada.” I am expressing a fact and I am sure and the degree of certainty is 100 %.
Must: It expresses a strong degree of certainty about an action in present time, but yet it is not 100 %.
Ex: Why isn’t Stephen in the class today? He must be on a trip to Canada.
Note. The speaker has a logical conclusion and some evidence to make him believe; he can be certain for 95 percent, not for 100.
50 % sure or less:
The modal auxiliaries like may, might and could express weak certainty.
Ex: Where is Stephen?
– He may be on a trip to Canada.
– He might be on a trip to Canada.
– He could be on a trip to Canada.
Note. The auxiliary can does not express any degree of certainty.
Ex: INCORRECT: He can be sick.
Negative form of degree of certainty in present time:
100 % sure: Stephen isn’t thirsty.
99 % sure: Stephen couldn’t be thirsty.
Stephen can’t be thirsty.
95 % sure: Stephen must not bethirsty.
50 % sure or less: Stephen may not be thirsty.
Stephen might not be thirsty.
- Stephen doesn’t want anything to drink. He isn’t thirsty. He told me that he drank a bottle of water before he came here. I heard it and I believe it.
- Stephen couldn’t or can’t be thirsty. It is not possible that the could be thirsty, because I just saw he drank water. I don’t believe it.
- Stephen doesn’t drink water. He must not be thirsty and that is the only reason that I can think of.
- I don’t know why Stephen doesn’t drink the water. He may not be thirsty or maybe he doesn’t feel well now.