I think some of the newbies are missing how to reply to a toot. You want to hit this button down here:
You'll see the edit section at the left (this is on a browser, not app) change to include the text of the toot you're replying to above the edit box.
This also works with your own toots, which is how you create threads (like I'm doing now)
If you decide against making the reply, hit the X in the upper-right corner of the quoted toot.