It is either part of the idiomatic tail
but also that I sometimes see omitted. Most of the time, you will, in fact, see
not only... but also perfectly intact, so I do not wish to disabuse you of its grammatical correctness. However, it is sometimes the case that you will see
not only... but..., without the
also, as though the
also were understood in the context. Even more rarely, you may come across a
not only... also sentence, as in,
He not only smells, he is also rude, making him a complete boor.I would suggest keeping the full idiom in mind when encountering
not only..., but, as with many grammatical rules, you ought to consider being flexible with your application of it.
I would be happy to answer any other questions you may have on the matter. Good luck with your studies.
- Andrew