Not Only But Also
Learn how to properly use the English pair not only and but also.
When not only is followed by but also (or simply but), it's considered good form to make sure the parts that follow each set of words are formatted the same way.
He is not only a great swimmer, but also a great musician. (Good: the sentence uses two noun clauses, which are underlined.)
He is not only a great swimmer, but also plays amazing music. (Bad: the sentence uses a noun clause and a verb clause. It's bad because they don't match.)
You could make the second example better by rewriting it with two verb clauses:
He not only swims with ease, but also plays amazing music.