Replying to a thread, will bring up an empty box for you to type in.

The previous posts on the thread (one topic---like this one) remain. You cannot alter someone else's post. Only moderators can do that if absolutely necessary or requested.

A thread is one topic with a starter post often abbreviated OP...original poster. It can be a question like you wrote and I moved here: A thread "grows" and depends on how many people respond to it.
http://neurotalk.psychcentral.com/thread162349.html
Click on this link and you will see your post that I moved here.

There are no responses to it yet. This is not a very busy venue, but the regulars will come on eventually in a day or so or more.

A post is one person's communication on the topic.

Just look around at the main index....
http://neurotalk.psychcentral.com/index.php

and when you feel comfortable about looking around at the various topics, etc there are more advanced things you can do.
http://neurotalk.psychcentral.com/thread37034.html
But to be honest, I just use the basic skills myself. This type of software offers lots of bells and whistles, but you don't need them to just basically communicate.

To learn a software, it is best to experiment with it. Click Back to go back, or click on titles to see them, anything underlined, or in a highlighted color is clickable (there are exceptions I guess but I've never found them.).

When you are done typing in a reply box, scroll down and hit Submit reply. If you goof up, one of us will see it and help you.
But it won't be instantly, but since this is a slower forum, a little wait is not a handicap.