AS I SEE IT....
In my view
Do you think people are more polite or less polite now than in the past?
when we draw the conclusion that something did NOT happen
must not have+ past participle
when we are certain that something was IMPOSSIBLE
couldn’t have + past participle
-Martha couldn’t have taken your notebook; she wasn’t even in class yesterday.
-The cookies are gone. But Eric couldn’t have reached the cookies on the top shelf; he must have asked his older brother to get them.
When someone is rude in a public place, do you say something or try to ignore the person?
[Have you] Seen my gloves anywhere?
She sang and [she] played the violin at the same time.
Complement