Mdtryhht

I wrote a sentence of the kind "XXX contains one of the most powerful lubricants that prevent rusty gears, the main cause of machine failure."
This is too wordy, of course, but never mind. The grammar authority marked this as needing singular instead of plural, prevent -> preventS, which I think must be wrong because I am trying to explain what these lubricants do, but there may be some other subtlety in the real example which I don't understand. Elsewhere I have seen stated that "one of the sisters" is singular not plural. A formulation of some general rules would be very much appreciated.

XXX contains one of the most powerful lubricants that prevent rusty
gears, the main cause of machine failure.

Verb agreement depends on whether the relative clause belongs in the embedded NP with the plural noun "lubricants" as head, or in the upper one with "one" as fused determiner-head.

If you're saying that there is a set of lubricants that prevent rusty gears etc, and XXX contains one of them, then plural "prevent" is correct.

The clause "one of the most powerful lubricants" can be interpreted as "a lubricant" (because you're picking one of them).

If you plug that into the original sentence...

XXX contains a lubricant that prevents rusty gears, the main cause of machine failure.

It's clearly singular and therefore "prevents" is correct.

Καλή σπέρα, Κώστα, καί καλως ηρθας στο ELU. The simplest way to see the matter is to consider this different way of organising the same sentence:-

1. Of the lubricants that prevent rusty gears, the main cause of engine failure, XXX contains one of the most powerful.

It is clear that this version means the same as the suggested correct version. But it is also clear that the suggested alternative is wrong:-

2. Of the lubricants that prevents rusty gears, ..., XXX contains one of the most powerful.

What is the difference? Well, version 2 makes it obvious the the relative pronoun that agrees with the plural noun lubricants and not with the singular noun XXX. If you wanted to have the singular, you would need to insert a comma after lubricants and (in British English, at least) the pronoun which.

3. XXX contains one of the most powerful lubricants, which prevents rusty gears...

Now, we can see that subject of the verb is the singular noun 'one'. Or you could rephrase it thus for clarity

XXX contains one of the most powerful lubricants and prevents rusty gears, the main cause of engine failure.

But I understood you to mean 2.

In your sample sentence, the subject and verb that are agreeing are "one" and "prevents".

XXX contains one of the most powerful lubricants that prevents rusty gears, the main cause of engine failure.

The prepositional phrase "of the most powerful lubricants" is there modifying the noun by defining the set of which "one" is a member.

The confusion comes because "lubricants", also a noun, occurs closer to the verb "prevents", and the eye and ear tend to group words by proximity, such that sentences with many embedded phrases cause some difficulty in identifying subject and verb. Consider adding two more modifying phrases to your original sentence and you'll see how difficult it is to maintain focus on subject and verb.

XXX contains one [of the most powerful lubricants] [on the market]
[for industrial farming equipment] that prevents rusty gears, the
main cause of engine failure.

Your grammar checker is probably just getting tangled up with the prepositional phrase in the same way that the ear can. This is inevitable.