だけじゃ〜ない grammar question

Question

I'm trying to figure out what this grammar is used for...

My textbook presents this:

a) 〜だけで、〜
b) 〜だけでは　〜ない
c) 〜だけじゃ　〜ない

I believe a) is just saying "by simply doing something/having something etc ~ I was able to do something..."

I believe b) is just saying "by simply being/doing something you can't also do/become/have something else"

I then have no idea what c) is trying to say. I don't know if it means by doing something you can also have something or if you do something you can't also have something else or what.... Please help!

Answer 1

Yes, your understanding of a) and b) seems fine.

As for c), it is no different in meaning to b), and the two are freely interchangeable. As in many other constructions (eg. the standard negative copula じゃない/ではない), じゃ is simply a contraction of では. The では form will generally be more common in writing and formal use, whilst the じゃ form is more common in spoken conversation.

A few examples of the structures in action:

裕福な家庭に生まれただけで、一生遊んで暮らすことができる。
Just by being born into a rich family, you can live a life of leisure.

お金だけでは解決できない問題もある。
There are some problems that can't be solved with money alone.

週に一回ジムに通うだけじゃ、痩せるとは限らないよ。
Just going to the gym once a week doesn't necessarily mean you'll lose weight.