The Prepositional Phrase Between Subject and Verb

"The box of nails are heavy" sounds fine to a lot of people because 'nails' sits right before the verb, and plural nouns pull for plural verbs out of habit. But 'nails' is stuck inside the phrase 'of nails', which only describes the box. The box is the thing that's heavy, not the nails. The verb has to agree with 'box', a singular noun, so the correct form is 'is'.

The fix is mechanical: find the preposition (of, in, on, behind, for, with, and so on), grey out everything from that preposition through the noun it attaches to, and look at what's left standing. In 'The list of ingredients printed on the label were inaccurate,' grey out 'of ingredients' and 'printed on the label.' What remains is 'The list were inaccurate' — and now the mismatch is obvious. 'List' is singular, so the verb must be 'was.' The noun inside the phrase, 'ingredients,' never had a vote; a noun inside a prepositional phrase can only ever be an object of that preposition, never the subject of the sentence.

This matters most when the test stacks phrases to bury the subject deeper: 'The recommendations in the report on the effects of years of drought on the valley aquifers have been adopted.' Four phrases sit between 'recommendations' and the verb, but you strip them all the same way, one after another, until only 'The recommendations have been adopted' is left. This skill is the backbone of every subject-verb agreement question that hides the subject behind description — once you can isolate the bare subject, you're ready for the harder versions where the distractor is a pronoun or a collective noun instead of a plain plural.