The Past Perfect and the Earlier Past

A sentence can hold two past events that happened at different times. Plain past tense ('arrived', 'spread') puts everything on the same level, so a reader has to guess which came first. 'Had' fixes that: it shoves one event further back in time, behind the other past-tense verb, which acts as the reference point. 'By the time the crew arrived, the fire had already spread to three buildings' tells you the spreading finished before the arriving started. Drop the 'had' and the sentence reads as if both happened together, which is the wrong picture.

Not every pair of past verbs needs this. If two events are simply told in the order they happened, plain past tense for both is correct and the past perfect would be an error: 'The technician sealed the tubes, labelled each one, and then shipped the crate' needs no 'had' anywhere, because the sequence is already obvious from the order of the clauses. The past perfect only earns its place when the sentence reports events out of order, or anchors a stretch of time that ran up until a later past moment, as in 'The timbers had lain under the silt since the vessel went down' before 'the ship was raised'.

Words like 'before' and 'after' already state the order, so some style guides allow plain past tense even with two past events: 'The fire spread before the crew arrived' is clear without 'had'. On the exam, when no such word does that job, the past perfect is the only thing holding the timeline together, and that is the detail the question is testing.