At the bottom of each post, see that little icon that says "quote"? When you see a person say something you want to reply to, you can click on the quote button under their post - it will open you a text box, quoting them, and you can type your response under the quote.

OR, when you're in the box where you're typing after you've hit the only reply button, you can scroll down and see the previous posts, so you can remember what or who to reply to. Previous posts are in order of the last post at the top, moving backward.

Hope this helps!

Sh'ra