Sequence of Tenses Across Clauses

A sentence builds one timeline, and every clause in it has to sit somewhere on that timeline without contradicting the others. If the main clause is in the past, a clause that depends on it usually has to stay in the past too, or even shift further back, because the two events are being located relative to each other, not described in isolation.

The clearest version of this shows up with reporting verbs like said, reported, or told. When the reporting verb is past tense, the thing being reported usually gets dragged back a step: present becomes past, past becomes past perfect (had + past participle). This is called backshifting. 'The foreman reported that the shipment had arrived three days earlier' pushes the arrival even further back than the reporting, because 'earlier' demands it. But backshifting is a habit, not a law: if the reported fact is still true now, the original tense can survive, as in 'The geologist told the students that the eruption occurs every two centuries,' because the cycle is still happening regardless of when it was reported.

To fill a blank correctly, scan outward from it for every tense clue: other verbs, time words like 'already,' 'earlier,' 'still,' and any dates or durations given. Those words fix a frame, and the blank has to fit that frame, not just sound fine on its own. This is the same logic that governs subject-verb agreement and parallel verb forms elsewhere in a sentence: grammar in one clause constrains the choices in the clause next to it.