In this excellent "modals review article [http://www.usingenglish.com/files/pdf/modals-review.pdf], on p. 4 Alex writes the following (shown below). In it, I'm not positive what he means by "PP". Does he mean probability/ possibility, or present participle, or past participle, or...? Help - thanks!
The past forms are:
must – must have + PP/ had to
can – could
can’t – couldn’t/ can’t have + PP
could – could have + PP
will – would
would – would have + PP
might – might have + PP
may – may have + PP
should – should have + PP
When there are two past forms, it is because they have different meanings, e.g. “couldn’t”
for ability and “can’t + have + PP” for probability/ possibility
Welcome to the forum, stddha.
It's the normal abbreviation of 'past participle'.