Last time I have started to mix active and passive forms like 読んだ｛よんだ｝本｛ほん｝ and 読まれた｛よまれた｝本｛ほん｝. In which case the passive is necessary and in which case not? Could you please help me to draw a fat red line between them? Thank you so much in advance...
Passive forms can be used when you don't want to (or cannot) clearly state the actor. Active forms like 「読んだ本」 can be used when the actor is clear from the context (but not so important as to be explicitly clarified).
So, 「読まれた本」 sounds like the book was read by an unknown person. On the other hand, the meaning of 「読んだ本」 should be determined from the context, like "the books I have read" or "the books you have read."
The spent money will not come back.
The money you have spent will not come back (to you).
In the former sentence, it is not clear who spent the money. It sounds like the money may have been stolen, or used without permission.
In the latter sentence, it strongly implies that "you" (or if it is a monologue, then "I") spent the money, because the actor should not be ambiguous.