Formation of Present and Past Modals
Present or future actions
When showing present or future time, the modal auxiliaries might, may, could, must and should precede the simple form of the verb, as in the example sentences below.
My friend got two speeding tickets last week. He must be a fast driver. (present)
The weather report said the temperature might drop below freezing level overnight. (future)
To form a negative sentence, place not directly after the modal.
She said she might not come.
A modal followed by be + progressive verb shows that the speaker thinks an action is happening at the moment.
Don’t make so much noise. My father might be sleeping.
When expressing past actions, might, may, could, must and should precede have + past participle, as in the following example.
The explosion was very loud. Even people living several kilometers away must have heard it.
To form a negative past tense modal sentence, use modal + not + have + past participle.
Tom is usually a good student , but he did poorly on the last test. He might not have understood the instructions.
To say you think an action was happening at a time in the past, use modal + have been + progressive verb, as in the next example.
The fire alarm sounded very early in the morning. Most residents of the building must have been sleeping at the time.
Meaning and Use
1. Might have, may have, could have
Might have, may have and could have are used to show past possibility. In other words, they are guesses about what happened at a period before now. See the examples below.
I can’t find my glasses. I might have left them in the car.
She promised to be here by now, but I don’t see her. She may have missed the bus.
The lights are all off in the house next door.. Our neighbours could have gone out.
2. Could have
Could have is also used to talk about something the speaker was able to do in the past but didn’t.
We were all having so much fun that we could have stayed longer. (We didn’t stay longer.)
I could have finished the report if I had stayed longer at the office. (I didn’t stay longer at the office. I went home.)
3. Must have
Must have is used to show probability in the past. That is, the speaker is almost certain that an action took place at a time before now.
There’s no milk in the refrigerator. We must have drunk it all.
Many more cars than usual were parked on our street last night. Someone must have had a party.
4. Should have, ought to have for advice
Should have and ought to have are used to express advice or offer an opinion regarding the past. It refers to an action that would have been good or appropriate, but never happened.
John missed the bus. He should have left his house a few minutes earlier. (He didn’t.)
John missed the bus. He ought to have left his house a few minutes earlier.
5. Should have, ought to have for expectation
Should have and ought to have are also used to express past expectation.
John should have been here by now. (I expected him to be here by now, but he isn’t.)
John ought to have been here by now.
6. Using “not”
The use of not with a past tense modal expresses the idea that whatever action took place was not good or appropriate. Consider the following examples.
I should not have drunk so much coffee. Now, I can’t sleep.
My suitcase is too heavy. I shouldn’t have packed so many things.