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I'm confused why the use of "increases" in the following sentence is incorrect --- I intuitively think it should be "increase" instead, but can't quite explain why I think so.

High consumption of trans fat is dangerous: not only does it increases your LDL cholesterol level, it also decreases your HDL cholesterol.

My confusion is exacerbated by the fact that "high consumption of trans fat is dangerous: it increases your LDL cholesterol level" seems grammatically correct, and the use of "decreases" in the sentence above also seems correct.

  • Yep, it's a tupo. – Hot Licks Nov 24 '18 at 3:07
  • Please cite the source. – Kris Nov 24 '18 at 8:33
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    @Kris --- are you referring to the source of the sentence I'm asking about? (If so, a friend sent it to me in an email). – Vincent Tjeng Nov 24 '18 at 16:40
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    Of course (but on other stackexchange websites I've generally waited a day before accepting!) – Vincent Tjeng Nov 25 '18 at 5:00
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You're quite right that it should be increase.

It's a basic rule of grammar that the verb should agree in number with its subject, which is why we use "is" (subject is singular noun phrase "high consumption of trans fat") and "decreases" (subject is "it") in the sentence.

However, the construction "not only does it increase your LDL cholesterol" is a bit deceptive, because the use of "not only" in a declarative sentence requires an inverted word order. The verb here is does, which as an auxiliary combines with the bare infinitive (in this case, "increase"). The subject is the singular "it", which because of the inversion comes after the auxiliary verb. The format is therefore:

Not only [auxillary verb] [subject] [bare infinitive] [object]...

Hence:

Not only [does] [it] [increase] [your LDL cholesterol level]...

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