Past Perfect

(a) Sam had already left by the time Ann got there.
(b) The thief simply walked in. Someone had forgotten to lock the door.
The past perfect expresses an activity that was completed before another activity or time in the past.

(c) Sam had already left when Ann got there.
In (c): First: Sam left.
Second: Ann got there.*

(d) Sam had left before Ann got there.
(e) Sam left before Ann there.
(f) After the guests had left, I went to bed.
(g) After the guests left, I went to bed.
If either before or after is used in the sentence, the past perfect is often not necessary because the time relationship is already clear. The simple pas may be used, as in (e) and (g). Note: (d) and (e) have the same meaning; (f) and (g) have the same meaning.

*COMPARE: Sam left when Ann got there. = First: Ann got there.
Second: Sam left.