The easiest way to quote is to highlight the text you want to quote and click the Quote button that pops up. This automatically inserts the proper code block and marks your post as a reply.
If you want to quote the entire post, it’s better forum etiquette to just click the Reply button on their post. This will show an indication on your post that you’re replying to theirs directly, and viewers can easily click it to view the original post (and vice versa). The only time this doesn’t show is when your reply is immediately after the post you’re replying to (such as my reply). The person you’re replying to will be notified in either case.
Edit: I’ve edited this post a few times because I was experimenting and had some slight misconceptions along the way. At first I thought that using the highlight+quote method did not mark one’s comment as a reply (with the mutual linkage), but it looks like it does, except in the case where your reply is immediately after the target post/comment. In any case, clicking Reply at any time on anyone’s post/comment will change the target of your existing reply and not wipe out your text which is handy.