Using What and Which

Question

In this two cases what we should use? What or Which?

1) what/which email program do you use

2) what/which accounts have this problem

Thanks.

Answer 1

Generally, when the response you want is singular, you should use "what" and when the response you want could be plural, use "which". For example;

What email program do you use?

Which accounts have this problem?

Answer 2

Generally "which" is used when there is a specific, identified set of choices. "What" is used when there is no such specified list. It's a little like the use of "the" versus "a".

So: "Everyone here must wear either a blue shirt or a red shirt. Which color do you want?" Versus, "You can wear any color shirt you like. What color do you want?"