equivalent to 'not only... (but also...)'; used to indicate that in addition to one thing, another also applies (somewhat formal/written).
彼は英語のみならず、フランス語も話せる。
He can speak not only English but also French.
この問題は難しいのみならず、解くのに時間もかかる。
This problem is not only difficult but also time-consuming to solve.
彼女は明るいのみならず、仕事もきちんとこなす。
She is not only cheerful but also handles her work properly.
used to add another related point; can mean 'furthermore' or 'moreover' when adding emphasis to an additional fact.
その政策は環境に良いのみならず、地域経済の活性化にもつながる。
That policy is not only good for the environment but also leads to revitalizing the local economy.
彼の発言は興味深いのみならず、問題提起として重要だ。
His remarks are not only interesting but are important as a point for discussion.
この機械は安いのみならず、使い勝手も良い。
This machine is not only cheap but also easy to use.