What's the most possible answer to this question?

Question

I wish you [could - would] stop smoking. I'm sure you will have better health.

This was a multiple choice question on my exam yesterday.

Answer 1

I wish you could stop smoking.

means "I wish it was possible for you to stop smoking." Perhaps the person is so thoroughly addicted that stopping is impossible.

I wish you would stop smoking.

means "I wish you would choose to stop smoking.

Thus either is possible depending on the intended meaning. Neither can be considered wrong or right without context.

However, many fluent speakers, particularly in informal speech, are not careful about this distinction, and may use "would" and "could" interchangeably.

Answer 2

In the first sentence, you can use either "could" or "would," though the meaning is different depending on which one you use.

I wish you could stop smoking.

is what I would say if I believe you are unable to stop smoking.

I wish you would stop smoking.

is what I would say if believe that you have the ability to stop smoking, but you choose not to.

Either way, the second sentence should use would, not will:

I'm sure you would have better health.