Go to the chapter of the comment you want to reply to. There you should see the comment section on the bottom of the sidebar. Between "All comments / Chapter comments" and the actual comments of the chapter you should see a field "Leave a comment..." Type your comment and click "Post" below it. To reach the comment field, you can also click the speech bubble symbol beside the Like button. (just above the chapter question)
I usually respond to comments via PM; I feel that the comment section of my story should have posts only from readers, not from me. But that's just my own opinion.
For other readers, this could look like you don't care anything about their comments, so if I answer with a pm, I usually leave a short comment anyway.
I know this sounds stupid but is their an expiry for comments. because I have notifications saying someone left a comment hell even a pm but when I check I got nothing
There is no expiration for comments. But comments can be deleted by the site administrator. This is mostly the case if there are spam comments or spam PMs. If the comment/PM is sent out, the notification gets fired. There might also be an email notification as well. Deleting these notifications might cause even more confusion, so we keep them. (I also think that it's not that easy to delete them.)
