I find it easier to quote someone, but this might help:
A guide to Discourse replies
By default, just reply to the last post. This is how conversations work. Someone says something, and you say something else in reaction.
Reply to a specific post (not just the last) if it will clarify your writing. In doing so, a reader needs to click only one button to see the post that most specifically prompted your idea. But remember, you are still speaking to the entire group, not just the single writer you are replying to. The Reply button is only a tool for adding context to your writing.
Another way of adding context to your posts is to use quotes. While writing you can highlight the text of any other post and it will be inserted as a quote in the body of your post. This way you can respond to many people and ideas in a single post . Quotes are awesome; use them.
If your idea was prompted by, but no longer serves, the conversation and the original topic, use the button on the right margin to reply as a linked topic. This is a great way to politely split off the conversation so that the original thread can stay on-topic, while still giving you the freedom to pursue a new conversation about a different idea. Participants will know the two topics are related thanks to links in the side margins. Interested participants will follow. And in fact, if a moderator feels that your post is better suited as a new topic, that moderator can retroactively gather your post and related replies into a new topic.
Above all, Discourse is designed to promote cohesive and rigorously on-topic conversations. By following these guidelines, it should be easy have meaningful conversations with far more elegance than a hierarchically-threaded forum can provide.