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This question already has an answer here:

I'd like to know whether "is" or "are" should be used in the following on standardized tests:

Reduced oxygen levels in the river is / are a problem for aquatic life.

I'd appreciate your help.

marked as duplicate by Michael Rybkin, Hellion, Jason Bassford, Davo, Chenmunka Feb 6 at 15:18

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    Ask yourself this: what is the head word of the NP "reduced oxygen levels in the river?". The head controls the number of the NP. – BillJ Feb 4 at 12:43
  • I saw a native speaker use "is." Was that a mistake? – Apollyon Feb 4 at 12:51
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The noun phrase here, forming the subject of the sentence, is "reduced oxygen levels in the river". There are three nouns here that could be the head word: oxygen, level, and river.

There are several approaches to working out which it is. You could see that "oxygen" is clearly qualifying "levels", or that the phrase itself breaks into two - "reduced oxygen levels" and "in the river", and that the second is clearly modifying the first. One trick is to see which parts can be missed out and still make sense - and that knocks out "river", as "in the river" does not need to be in the sentence. It makes sense - albeit ambiguous sense - without that phrase: "reduced oxygen levels is/are a problem for aquatic life".

Next you can think about how you would rephrase it. How could you rephrase "oxygen levels"? What's another way of saying that? Well, the one that makes sense, in context, is "levels of oxygen". "Reduced levels of oxygen in the river is/are a problem for aquatic life. That tells us that "levels" is the head word. Levels is plural, so the noun phrase "reduced levels of oxygen" is plural, so the correct choice is are.

There's two reasons even a native speaker might get confused. Sometimes we intuitively understand how a noun phase is plural or not, but sometimes people just latch on to the last noun in it, especially if it's the last word in the phrase, and use the number of that. They mentally construct it, see that "river" is the last word before the verb, and thus conjugate "to be" based on a third person singular.

Another is that this could be phrased different so that it would be singular, because 'levels' is actually redundant. However, if 'levels' is present, it is the headword. If you say "reduced oxygen in the river is a problem for aquatic life", that's fine. So is "reduced oxygen levels in the river are a problem for aquatic life". The two versions can get confused in the mind of the speaker, even a native speaker, and lead to mistakes.

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The levels are a problem.

Reduced and oxygen are just there to qualify to word levels, which is the real subject of the verb

An atmosphere with reduced levels of oxygen is a problem.

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