「わけがない」と「わけではない」

Can someone explain to me the subtle differences between 「わけがない」と 「わけではない」?

From what I gather, わけがない is equal to 可能性がない, whereas わけではない is less of an impossibility. Is this correct? Also, could someone please write an example sentence or three?

Here are a few of mine:
わけがない：
この町を知らないから、正しい道路を分かるわけがない よ！
I don't know this town so how should I know the correct road!

わけではない：
ここではほとんどの人は日本語がしゃべられるのに、全 て話せるわけじゃないね。
Most people here speak Japanese, but that doesn't mean that all do, you know.
*Can I use わけじゃない as a less formal version of わけではない？

Much obliged!
J

わけ（訳） means reason, logic, sense, etc.

（１）わけがない
There's no reason it could...; there's no logic it can....;
It denies the possibility entirely.

こんな難しい本は初心者に読めるわけがない
There's no reason/logic that beginners can read such a difficult book
→ Beginner couldn't read such a difficult book.
私の気持ちがあなたにわかるわけがない
There's no reason/logic that you could understand my feelings.
→ You couldn't understand my feelings.

Similar expression is はずがない
(はず means prospect, expectation, etc.)

（２）わけではない
It does not always mean...; it does not immediately mean...; it is not a matter of course
(Maybe it is easier to get the nuance of it if you take わけ here as a situation including both the reason and the result.)
曇っているからといっていつもその後に雨が降るわけではない。
Even when it is cloudy, it does not always mean it will rain.
英語が話せても仕事ができるわけではない。
He can speak English but it does not mean he's competent at work.

Yes, わけじゃない is more colloquial than わけではない.
じゃ is contraction of では.

Similar expression is
～とは限らない（～とはかぎらない）
限る here means specify, limit.
英語が話せるからといって仕事ができるとは限らない。

I cannot think of good/enough examples... hope others will add some more.

jljzen said:

Can someone explain to me the subtle differences between 「わけがない」と 「わけではない」?
From what I gather, わけがない is equal to 可能性がない, whereas わけではない is less of an impossibility. Is this correct? Also, could someone please write an example sentence or three?
Here are a few of mine:
わけがない：
この町を知らないから、正しい道路を分かるわけがないよ！
I don't know this town so how should I know the correct road!
わけではない：
ここではほとんどの人は日本語がしゃべられるのに、全て話せるわけじゃないね。
Most people here speak Japanese, but that doesn't mean that all do, you know.
*Can I use わけじゃない as a less formal version of わけではない？
Much obliged!
J

Click to expand...

その赤ちゃんが、そんなことを言うわけがない。
The baby can not say so.
そのことについて全部を知っているわけではない。
I don't know all about it.
わけじゃない is ok.