Currently, email subscription window looks like this:

I think the amount of information about the results of the subscription right now is not enough. My suggestion is to have an info button next to the text, which will show an explanation like the following:

Subscribe to all replies to your comments and all discussions under them.

About the form, it can be a small non-modal window or just a text box which shows under the current text in the same box and hides by clicking a cross button.