When two verbs took place in the past, the first completed one is put in the past perfect whereas the second is in the past simple.

eg. I had arrived befor he left.

But, I found a sentence in Practical English Usage by Swan which is:

I arrived before she had finished unpacking.

Would you elaborate on that?

In your example sentence, Ismael, it is clear that the first completed action is your arriving, so the action of that is in the past perfect tense. The second action is his leaving, so this action is in the simple past tense.

In Swan's example, the past perfect – had finished – emphasizes the idea of completion. With "before," "had finished" actually refers to the second action, which is not complete.

Swan has a similar sentence with a note:

"He went out before I had finished my sentence. ( "...before the moment when I had completed my sentence.)

(Note that in sentences like the last, a past perfect tense can refer to a time later than the action of the main verb. This is unusual.)"
_______

I think the logic of the tenses in this kind of sentence is this: You arrived. You would have expected her to have finished packing when you arrived. The expected first action was that she finished packing, even though that didn't actually happen. Because that action was expected to be the first completed action, its verb is in the past perfect. "Before" + "had finished" was supposed to have happened first, but we can tell from the sentence that it really didn't.

"Before" + "had finished" appears frequently with this meaning: the first action is not having finished, and the second action is something else. Examples from the New York Times:

"¢ ..the splintered rhythm, the harmonic palette -- was influenced by Mr. Osby, who hired the young pianist before he had finished his music degree. But more recently Mr. Moran and his group have been taking those lessons (and also the idea...

"¢ ..sometimes pages upon pages of additions, entire new developments, alternative drafts. Since death arrived before he had finished correcting the final volumes, it's never been entirely certain what material was to be considered definitive...

"¢ ..kill?'' Mr. Salaam denied the prosecutor's charge. But as he often did yesterday, he began his answer before she had finished her question. So it was the question and not the answer that seemed to have been the last word.

"¢ ..pro-American crowd was even more pro-excellence. Like Miss Zayak, Miss Biellmann drew a standing ovation before she had finished. And like Miss Zayak, she drew high marks from the nine judges. There were no perfect scores of 6.0, but,,,

Rachel