This is a good idea.

这是一个好主意。

That's not a good idea.

那不是个好主意。

Yes, that's a good idea.

是的，这是个好主意。

Indeed, it is a good idea.

的确，这是个好主意。

I think this is a good idea.

我觉得这是个好主意。

That sounds like a good idea.

听起来是个好主意。

Doing more exercise is a good idea.

多做运动是个好主意。

It's a good idea to save for a rainy day.

未雨绸缪是一个好主意。

I don't think so, although it's a good idea.

我不这么认为，虽然这是个好主意。

But it isn't a good idea because there are cars.

但这不是一个好主意，因为有汽车。