いいじゃない。vs いいのがない。

Question

I was watching this video about conversations in Japanese. There's a couple discussing about what apartment they should buy...

Wife: そうねえ、じゃ、これはどうかしら？

Husband: なかなかいいんじゃない。これ見に行ってみるか。

W: あ、ちょっと待って。でも、これ、クローゼットがちょっと狭いわね。

H: 確かに。う～ん。なかなか、いいのがないなあ。

So, here lies my question, Why the first one using いいじゃない。sounds like rhetorical no, therefore an "Yes"? But the second sounds like a negative　(I'll put the translation given by the video)

Husband: なかなかいいんじゃない。Yeah that's nice. (isn't it)

H:いいのがないなあ。 There doesn't seem to be the one that is perfect.

So Why the first is a positive answer and the second a negative? Maybe because the first sounds like a question? I'd say that there is an question intonation to it, but in the writting there's no indication if it's was supposed to be a "question"(rhetorical) or not...

Source: https://youtu.be/xVsOB9LiV7A?t=1m55s 1 minute 55 seconds

Answer 1

There's not really any way of interpreting なかなかいいじゃない as anything but a rhetorical negative. Firstly, you would never ordinarily make いい negative by suffixing じゃない - it would always change to よくない. So this makes it clear that the じゃない is a rhetorical feature affixed to the sentence more generally rather than a negation of いい. And secondly, なかなか generally has a positive nuance, so it wouldn't tend to be used with an actual negative.

There are indeed similar constructions that would be legitimately ambiguous - for instance, if the phrasing was 素敵じゃない, it could be interpreted as either an actual negation "it's not lovely" or a rhetorical tag question "isn't it lovely" depending on the tone of voice. In that case, you'd have to rely on the surrounding context to determine which meaning is intended in a written dialogue.

In practice, though, this would be quite easy to distinguish, since a negative 素敵じゃない would only be used if someone had just suggested that the house was 素敵 (and would often be preceded with a まったく or similar), whereas the much more common positive 素敵じゃない would generally be the first mention of 素敵 in the conversation (since it would be strange to follow up "it's lovely" with "isn't it lovely!"), and might be preceded with positive expressions like へ～ or なかなか.

As for いいのがないなあ, this can't really be interpreted as a tag question, because the ない is a stand-alone verb meaning "there isn't any" rather than a じゃない, and the なあ makes it clear that this isn't a question. If it was intended with a positive meaning, you'd see something like いいのがあるじゃない。

Answer 2

So Why the first is a positive answer and the second a negative?

It's because the first one is a set phrase. It cannot be negative. It's a kind of an "idiom." So there is no confusion among native speakers.

Maybe because the first sounds like a question?

You're correct. It is also expressed as ”いいじゃないか”。or　”いいじゃないか？”　or "いいではないか”.

I'd say that there is an question intonation to it, but in the writing there's no indication if it's was supposed to be a "question"(rhetorical) or not...

Even in the writing, native speakers almost simultaneously or instinctively can understand that it's rhetorical. Because it simply doesn't make sense and weird if one interprets it as a usual negative sentence. It should be something like: 良くない（ね）。いいとは言えない（ね）。

Native speakers would choose different sentences to mean negative. So there is no confusion.　Hope this helps!