What is the difference between the these two sentences, and which one is correct?
(1)He passed the exam because I gave him my book (2)He passed the exam because I had given him my book
On the surface, there is no difference between these sentences. If I were to encounter one or the other on its own, I doubt that I'd take them to mean different things. However, if I had to contrast them directly, there are some subtle differences.
Because the past perfect implies that the things in question is complete, I might take "I had given him my book" to imply that- I now have the book and I did not give him. Or I might take it to imply that- I gave him the book in advance of the exam. Although the simple past doesn't exclude either of these scenarios, it doesn't imply them, either.
If I had to take it a step further, I'd also suggest that the perfect past in this instance very subtly elevates that part of the sentence - makes my having given him the book seem more significant. In the simple past version, there is equal emphasis on his having passed the exam and my having given him the book.
The past perfect one is correct of course. The past simple one I think is correct too.
Although my grammar book said:
If you don't want to emphasize the causal relationship, you can use past simple to express the event happened before a past point. (I translated myself)
I still got a lot of sentences from novels, these are quotes from them:
Jonathan Small did not get the treasure because he and his associates were themselves convicts and could not get away. The Sign of the Four
This was overlooked because it was in the darkest corner of the room, and no one thought of looking there. A Study in Scarlet
We called him Tortoise because he taught us. Alice's Adventures in Wonderland
I believe in these authors.