Present Perfect vs Past Perfect

PRESENT  PERFECT The present perfect is used to refer to actions which take place in an unfinished time period up to the time of speaking, and allows the speaker to link past actions or situations to the present time.  Continuous form :  Actions started in the past which continue until now. I have been reading this book since last Monday. Simple form : The completed or finished part of a continuous action.  I have read the first three chapters of the book. Events that have just occurred, with no specific time mentioned. A plane has just crashed near the coast. A past action with a result in the present. I've broken my arm.  I can't drive. Experiences and accomplishments up to now. I'm a writer. I've written 5 novels and several short stories.

PAST PERFECT

If we are already talking about the past, the past perfect is used to to go back to an earlier past time,  to refer to something that had already happened or had been happening. Continuous form : Continuous actions entirely situated in the past. Paul had been revising his English when the postman rang the doorbell. Simple form  : .Actions which took place before a specific time in the past. When Tom called at 8 am, Paul had already left for school.