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In the first post on modality, I only covered the forms referring to a future activity:
We should leave soon.
We could go to the cinema.
She might come tomorrow.
You can/may go now.
I would like to go now.
I will go now.
We can also use modality with past time reference if we need or want to.
Let’s say e.g. I told someone to go to the dentist yesterday, but she didn’t and now she has a bad tooth ache. I gave her the advice to go, and now I want to rub her nose in it that she didn’t follow my advice:
You should have gone to the dentist (as I told you).
With the advice itself, time is not really an issue and not expressed specifically by the modal (as modals don’t change their form in any respect they don’t express anything but what their semantics allow), and the validity of my advice actually still stands – but the activity of the verb (go to dentist) did not happen.
The form of a modal construction with past time reference is always the same: modal auxiliary + have + (past) participle.
They could have gone (earlier) – but they didn’t
We might have helped (if they had asked us) – but we didn’t
In the negative form, ‘not’ (n’t) is added to the modal:
They should not have gone – but they did
or with ‘could’:
You couldn’t have helped – even if you had wanted to, their was nothing you could have done.
He might not have done it – had the circumstances that led to him doing whatever he did been different.