だけでなく to Express "Not Only..."

The structure [Noun/sentence + だけでなく, Noun/sentence も] can be used to express something like "Not only X, but also Y." The だけでなく portion can be either that or だけじゃなくて. The second noun or sentence can also have a particle paired with it before the も.

京都は古いお寺があるだけでなく、きれいな景色でも有名です。
Kyoto not only has old temples, but it's also famous for its beautiful scenery.

このアパートは駅から近くて便利なだけじゃなくて、家賃も安いから、借りることにした。
Not only is this apartment close to the station and convenient, but it also has cheap rent, so I decided to rent it.

その映画は面白いだけじゃなくて、音楽もいいよ。
Not only is this movie interesting, but it also has good music.

日本語はひらがなだけでなく、カタカナや漢字も覚えなくてはいけません。
With Japanese, you must learn not only hiragana, but also katakana and kanji.

One note about that last sentence there. Notice the 覚えなくてはいけません part? I was initially confused because I mistook it for the grammar used to say "you must not do X." In reality, it means "you must do X." Why?

Verb's て form + はいけません = "You must not do X"

Verb's negative て form + はいけません = "You must do X"

Confusing, isn't it? The double negative results in the positive meaning for "you must do." It's just like the other "have to" verb conjugations I learned in Genki.