だけでなく and だけじゃなくて

Question

According to the tobira textbook, ~だけでなく、~も~, means "not only ~, but also ~". One example they give is, 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。

For the last two examples, the grammar phrase being taught changes to ~だけじゃなくて、~も~. The examples are

• 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。
• その映画は面白いだけじゃなくて、音楽もいいよ。

Do だけでなく and だけじゃなく both have the same meaning here? I'm assuming they are the same or similar but I'm not sure. Is one a less formal version or the other? Are they identical?

Also, in my second example, why is 便利 followed by なだけ instead of just だけ? Is だけ perhaps a noun that 便利 is modifying?

Answer 1

The difference between でない and ではない is directly connected to the semantic impression the topic particle は would give in normal circumstances.

ではない appears to be generally preferred in distinct, independent predicative phrases, or when you are about to say "Something 'is' something, and so that...":

• このアパートは駅から近くて便利なだけじゃなくて...

  "This apartment is not only convenient being near a station..."

• その映画は面白いだけじゃなくて...

  "The movie is not only interesting..."

• 人間たちが定命の者ではなかったら、間違いなく神にもっとも近い種族だったでしょう...

  "If human beings are not who lay down fate..."

Whereas でない can be seen employed in other environments including but not limited to:

1. objects dependent on a verb phrase:

   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...

   "For Japanese you have to learn not only hiragana..."

2. direct attributives which modify nouns:

   観光コースでない沖縄

   嫡出でない子

   This type is rather straightforward.

3. When directly preceding, especially having an immediate causal connection to, the main event/verb of the sentence:

   ママでなくてよかったよ...

   "Good that it wasn't mom...". In Japanese grammar context this use is closely related to the one above since adjectival/adverbial constructions are viewed as parallel, but such adverbial ones often lack a literal English parsing. In this case, *ママではなくてよかった would have meant something along the lines of "It wasn't mom, but (I was? That thing who wasn't mom was?) good" which doesn't make any sense since は would have elevated that part of the phrase to an independent level parallel to the main event よかった. To use は in this context, you have to add extra conjunctions and say for example ママではなかったからよかった.

   ヒールでないとダメ...

   Similar to above. ヒールでなくダメ　can also be correct, with the former putting more emphasis on a general case "if" and the latter more of a specific instance such as when a woman is stopped right before the entrance of a high-heel only venue.

   and one final example:

   試験は優秀でないと合格できない...

   which is almost exactly the same as the one above.

4. isolated phrases where the integrity of a full sentence is not as emphasized; frequently in titles:

   愛でなく

   感覚でなく思考

Answer 2

• だけじゃなく is an informal/casual form of だけでなく.
  
• It's not "便利 + なだけ", but "便利な (na-adjective) + だけ".

Answer 3

I'll add an answer to your second question, to clarify Doncot's answer which feels rather partial.

じゃない is indeed a casual form of でない

When used to modify a whole sentence, だけ attaches to the rentaikei of the previous word. That's a fancy way for saying that if a sentence has だ at the end, it should be replaced with な:

• verbs: このボタンを押すだけでいいよ。 (no change)
• i-adjectives: 話し相手がほしいだけだ。 (no change)
• nouns and 'na-adjectives': 必要なだけのお金を持ってきて。

The word also attaches to な-adjectives with the な intact: 好きなだけ