She not only plays the piano, but also the violin.

[stephenlearner]

She not only plays the piano, but also the violin. (from a grammar book)

Is this sentence correct? I don't think so. Because not only in the sentence governs the first phrase, not the second phrase. That is to say, the second phrase doesn't have a verb, and possibly a subject.

I would say

She plays not only the piano, but also the violin. or

She not only plays the piano, but she also plays the violin.

 

[Uncle Jack]

It is common enough in speech, inasmuch as "not only...but also..." is used at all (I don't think it is nearly as common as some books make out).

"She not only plays the piano..." is fine, and sounds far more natural than "she plays not only the piano...". However, for a parallel structure, the verb needs to be repeated. I expect that most people who use this construction would repeat the subject and verb for emphasis, since emphasis is why this construction is used in the first place:

She not only plays the piano, she also plays the violin.​

Where the subject is repeated, the conjunction is omitted.

 

[Loob]

I agree with you, stephenlearner. In a grammar book, I would have expected
She plays not only the piano but also the violin.