You may add a comment or reply to a post, unless comments have been disabled.
You can also comment or reply to a previous comment. To do so, click on the Reply ↓ button at the end of the comment to which you wish to reply. This will associate your reply with that one, rather than the original post.
When there are several replies or replies to replies it helps if you make sure your reply is linked to what you are replying to.
By default you will be subscribed to receive a notification when there is a reply to your comment.