grammaticality - Use of 3rd person present tense when combined with "not only"

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.

Answer

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]...