だけでなく details
Examples
not only N — extends a set to include more than the noun named, usually paired with も
この店、安いだけでなく味もいいんだよ。
kono mise, yasui dake de naku aji mo ii n da yo.
This place isn't only cheap — the food's good too. casual
彼は英語だけでなくフランス語も話せます。
kare wa eigo dake de naku furansugo mo hanasemasu.
He can speak not only English but French as well. neutral
今回のセミナーは学生だけでなく社会人の方にもご参加いただけます。
konkai no seminaa wa gakusei dake de naku shakaijin no kata ni mo go sanka itadakemasu.
This seminar is open to working professionals as well, not just students. formal
not only [predicate] — extends a state or action described by a whole clause
この漫画、面白いだけでなく、勉強にもなるんだ。
kono manga, omoshiroi dake de naku, benkyou ni mo naru n da.
This manga isn't only entertaining — it's actually educational too. casual
この機械は使いやすいだけでなく、値段も手頃です。
kono kikai wa tsukaiyasui dake de naku, nedan mo tegoro desu.
This machine isn't only easy to use — it's reasonably priced too. neutral
本製品は高性能なだけでなく、省エネルギーでもございます。
honseihin wa kouseinou na dake de naku, shouenerugii de mo gozaimasu.
This product is not only high-performance, but energy-efficient as well. formal
Formation
{N}だけでなくattaches to nounplain clause + だけでなく（な-adjは なだけでなく）attaches to plain clause
Notes
a bare だけでなく with nothing added back sounds unfinished, much like
によると without a hearsay tail.
Related patterns
Similar
Near-identical "not only X, Y too" frames, but だけでなく is the register-neutral default in both speech and writing while ばかりでなく skews written/formal and sounds stiff dropped into casual chat.
Lookalikes
だけでなく contains だけ but points the opposite way — bare だけ closes a set down to exactly N (exclusive), while だけでなく opens it back up to at least one more thing (additive); 学生だけ excludes everyone else, 学生だけでなく社会人も招く includes more people.