The perfect progressive tenses

Form: have + been + -ing (present participle)
Meaning: The perfect progressive tenses give the idea that one event is in progress immediately before, up to, until another time or event. The tenses are used to express the duration of the first event.

PRESENT PERFECT PROGRESSIVE

(a) I have been studying for two hours.

Event in progress: studying. When? Before now, up to now.
How long? For two hours.

PAST PERFECT PROGRESSIVE

(b) I had been studying for two hours before my friend came.

Event in progress: studying. When? Before another event in the past.
How long? For two hours.

FUTURE PERFECT PROGRESSIVE

(c) I will have been studying for two hours by the time you arrive.

Event in progress: studying. When? Before another event in the future.
How long? For two hours.


Cheers!
Naeem