When an Intervening Phrase Suggests a Different Number

A sentence's main verb has to match its main subject in number, but English loves to shove extra material between them: prepositional phrases, phrases set off by commas, relative clauses starting with 'who' or 'that'. That gap is where mistakes happen, because the noun sitting right before the verb often has a different number than the real subject. 'The list of approved vendors' looks plural because 'vendors' is the last noun you read, but 'list' is the subject, and 'list' is singular. Cross out 'of approved vendors' and the sentence reads cleanly: the list is updated.

Relative clauses work differently, and this is where the rule gets tricky. A clause starting with 'who' or 'that' doesn't take its number from the main subject of the sentence; it takes its number from whatever noun it directly attaches to. In 'one of the engineers who have reviewed the flood model,' the clause 'who have reviewed' attaches to 'engineers,' not to 'one,' so it needs the plural 'have.' Watch what 'only' does to this pattern: 'the only one of the runners who has finished' flips the attachment back to 'one,' because now there's just one runner the clause can describe, so the verb goes singular.

The same skill of mentally deleting the interrupting phrase shows up when you're checking whether a modifier is placed correctly. Strip the phrase, read the bare bones, and both the agreement and the logic of the sentence become visible.