What should follow “I'll tell you another reason…”?

Question

Is any of these the correct form of the sentence?

"I'll tell you another reason *I will not participate"

"I'll tell you another reason why I will not participate"

"I'll tell you another reason for which I will not participate"

If not, what is it? What's the grammar behind it?

Answer 1

why and for which both make sense, however. For which sounds more formal, and looking at the context of the sentence, this is probably what you want to be going for.