As long as you are a registered user and you are logged in, you will get the "Quick Reply" box at the bottom of each thread.
You can choose to reply within this quick reply box (which has many of the necessary features), or you can use the "Go Advanced" button and have all the same features as you have when you start a thread.
The benefit to the quick reply box is that you can still view the thread as you are posting, whereas with the advanced post you cannot.
Then hit "Post Quick Reply" if in quick reply mode, or "Submit Reply" if in advanced mode.
You as well as anyone else who has responded to this thread will receive an email notification letting you all know who responded to the thread.