To show that we had the ability to do something in the past, but in fact we didn't do it.
I could have taken the exam, but I chose to skip class instead.
When sensible advice wasn't followed or when expectations or obligations weren't fulfilled.
You should have done the test when the class did. Now you'll have to stay after school.
When we are guessing because we don't know what happened in the past.
Fritz hasn't been out of his room all day. He might have been studying.
When we are quite certain about things that took place in the past, or to draw logical conclusions.
Helga didn't come to my party last night. She must have forgotten.
To express an action which we were welling to do in the past but which in fact we didn't do.
I would have come to school earlier, but my car had a flat tire.
All the modal perfects refer to past actions which did or did not take place. The meaning of the sentence varies according to the modal used.
Does Mr. Herz speak German?
Yes, Mr. Herz speaks German.
Is Diane out sick?
Yes, she is out sick.
Doesn't class start at 10:30?
Class does not start at 10:30. Rather, it starts at 10:25.
Doesn't everyone know that sugar is a poison?
Of course. Everyone knows that sugar is poison.
Who is trying to take attendance?
Mr. Herz is trying to take attendance.
Where is Diane resting?
Diane is resting at home.
Is it raining now?
It is not raining now.
Who is getting together to practice English today?
We are all getting together to practice our English this afternoon.
When did Mr. Herz marry?
Mr. Herz married in 1997.
Who came to school late today?
I came to school late today.
When did class start?
Class started at 10:25.
What idiot thought that fat makes you fat?
Ancel Keyes thought that fat makes you fat.
When will Mr. Herz learn to code better?
He will learn to code better when he practices more.
When will Diane return?
Diane will return next week.
Will class end late today?
Yes. It will end late.
Will I see you next week?
No, you will not see me next week.