Is there a way to respond to a post?

Are you using the website version or the app version? Someone else will have to talk about using the forums on the app, but on the website when you open a discussion thread and scroll to the very bottom there will be the words "Leave a Comment" with a box below. Click in the box, you can start typing. When your post is ready, click the blue box "Post Reply".

If you want to respond directly to a specific post, you can click on the word "quote" beneath the post in question, the website will automatically jump you down to the "Leave a Comment" box along with the quoted comments of the post you want to which you wish to respond. You can leave the entire quote, or you can edit out the parts which do not concern your response (for example, a quote talks about both running shoes and shorts, but you only want to discuss shoes, you can delete the part about the shorts).