not only… but also
Used to say 'not only A but also B', adding an extra item or aspect beyond the first one mentioned.
だけでなく is a fixed expression that means 'not only... but also'. It is used to link two items or statements, emphasizing that the second one is additional and often more important or surprising. For example, you can say 彼は英語だけでなく日本語も話せる to mean 'He can speak not only English but also Japanese.'
In terms of form, when you attach it to a noun or な-adjective, you just put だけでなく after it. For verbs and い-adjectives, you use the plain (dictionary) form before だけでなく. The second part of the sentence often includes も (also) to match the 'also' meaning, but it is not required.
This expression is used in both spoken and written Japanese, but it sounds a bit formal or explanatory. In casual speech, you might hear だけじゃなく (a contraction of だけではなく), but that is more colloquial. For N3 level, using だけでなく is safe and appropriate in most situations.