The English verb system can be devilishly difficult, my favorite example being this "conjugation" of watch:
She must have had been being watched.
= It must be the case that someone watched her for some duration of time prior to the current moment in the story (which is either a hypothetical story or narrative of the past).
She must have had been being watched.
= It must be the case that someone watched her for some duration of time prior to the current moment in the story (which is either a hypothetical story or narrative of the past).