When defective verbs are followed by have + past participle, they indicate a past action.
That action was not fulfilled or other way round.
| could | been | |
| may | done | |
| might | gone | |
| should | + have + | asked |
| must | begun | |
| ought to | worked | |
| need | bought |
Examples
I could have bought the mobile phone if I had thought of it.
If you thought of it, you could have done the job today.
You might have let me know.
You should have finish that earlier.
The verbs could and might in sentences, referring to the present time are modal verbs.
Examples
I could come tonight if you need me.
I could not promise anything about that.
You could finish this job.
She could make a cookies.
Could is also used to express a polite request.
Examples
Could you please, tell me what time it is ?
Could you give me a hand, please ?
Could I ask you a favour ?
