not only ... (but also)
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
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
Variants
だけではなくdakedehanakuformal
inserts topic は for a fuller, more explicit negation — reads slightly stiffer/written, the safer pick in reports and announcements
だけじゃなくてdakejanakutecolloquial
では→じゃ is the everyday spoken contraction, plus て to run into the next clause — the real casual default, more natural than bare だけでなく in brisk conversation
だけでなく
だけでなくdake de nakuN3neutral