Generally speaking, quoting from previous posts isn’t necessary in Discourse. Our discussions are generally pretty short, so it’s easy to figure out what was said before and what you’re replying to. However, in some cases, a short quote can give your reply some helpful context.
To quote text from a previous message (and you can do this multiple times, from multiple previous messages), simply select the text and click the gray Quote button that appears.
Discourse responds by quoting the selection in your reply, so you can type right after it.