I read in an English grammar book "I have found the keys which I had lost". Why is past perfect being used? Shouldn't it be "I have found the keys which I lost".
In English, you need to use past perfect tense if the order of events isn't clear.
When she arrived, the party started.
When she arrived, the party had started.
In the first sentence, she arrives, and the party starts simultaneously. In the second sentence, the party starts, and then she arrives.
We generally don't use the past perfect if the order of events is clear, and the verbs are in the same order that they occur.
I lost my keys, but I found them.
I had lost my keys, but I found them. (Grammatical, but less idiomatic.)
If the order of events is clear, but the verbs aren't in chronological order, you don't need to use past perfect, but people often do.
I have found the keys which I had lost,
I have found the keys which I lost,
are grammatical and idiomatic. (Although unless they're emphasizing one of the verbs, native speakers are more likely to say I've and I'd in those sentences.)