I could have been relaxing at home if I didn't have to work today.
They should have been doing their homework at the
time that I called.
He would have been working at that company if he hadn't been laid off.
You might have been thinking I was much older.
This is a diffficult modal form for beginning
and intermediate level students. In this case, you're describing
something that was possible in the past; however; another event
emerged and forever altered the past. Don't worry about mastering
this right now, but listen to how Americans use it--whether
correctly or not.
This penquin could
have been swimming in Antarctica
today if he hadn't been captured there two years ago.
They should have
been taking a test but instead they skipped
school and went to a malt shop.
He would have been
working in a bank
if he hadn't decided to become a beekeeper.
He might have been
thinking it was earlier. Now he realizes he's late
for a meeting.
(This helps describe problems with perception of time.)
To make the negative, add "not" to the modal:
I shouldn't have been working so late last
night. (but I did)