how/when do i use had and have/has?
what is the difference?
she has read the book
she had read the book
thanks in advance...
 She has read the book before.
 She had read the book before she saw the movie.
With has read, the present perfect, the event, not when the event took place, is in focus. Haven't you ever forgotten when exactly you did something or when you did it wasn't important at all in what you wanted to express? That's when you use the present perfect. When the time you did it isn't important. In  she read the book. That's the important part, not when she read it. If you want to talk about when, use the past simple; e.g., she read the book last week.
With had read, the past perfect, that event happened before another event in the past. The past perfect functions like a bridge. It connects two events in time. So, in  she had read the book before she saw the movie:
The event that happened first: read book
The event that happened second: saw movie
Does that help?
Didn't see you there, Harry. We must have been posting/typing at the same time.