We can use a modal verb with have and a past participle:
|Subject||Modal||have||Past participle||Adverbial, object or complement|
|Jack and Jill||would||have||been||late.|
We use a modal verb with have:
- to refer back from the present:
It's nearly eight o'clock. They will have arrived by now.
- to refer back from a point of time in the past:
We were very worried. We thought someone might have taken the car.
- to refer back from a point of time in the future:
We won't eat until they arrive. They might not have had supper.
- to refer to past time:
You should have helped her when she asked.
They might have got lost. Nobody knows where they are.