What it means
だけでなく～も means “not only A but also B.” It adds B to A and emphasizes that both apply. The first part can be a noun, a na-adjective with な, an i-adjective, or a plain-form verb. The second part is commonly followed by も, and the sentence predicate can be familiar or polite.
Usage note
だけでなく～も usually connects two parallel items or facts. The familiar or polite style comes from the rest of the sentence, especially its final predicate; だけでなく itself does not change.
How to use it
このかばんは軽いだけでなく、肩にもやさしいです。
This bag is not only light but also gentle on the shoulders.
この図書館は本だけでなく、雑誌も置いています。
This library has not only books but also magazines.
田中さんは英語を話すだけでなく、中国語も教えています。
Tanaka not only speaks English but also teaches Chinese.
この公園は静かなだけでなく、子供にも安全です。
This park is not only quiet but also safe for children.
このかばんは軽いだけでなく、肩にもやさしい。
This bag is not only light but also gentle on the shoulders.
この図書館は本だけでなく、雑誌も置いている。
This library has not only books but also magazines.
田中さんは英語を話すだけでなく、中国語も教えている。
Tanaka not only speaks English but also teaches Chinese.
この公園は静かなだけでなく、子供にも安全だ。
This park is not only quiet but also safe for children.