Here's a picture:
What's malformed? Notice that his indent level is the same as the quote.
If you find yourself typing on this "indented" level you've screwed up. It's okay. Stuff happens.
But please fix it before you submit the text, because if we see malformed posts that take more than a few seconds to fix, we'll delete them instead. If you don't know the HTML (and don't want to mess with "Source" mode), then discard your draft and re-quote posts and try again.
TL;DR: if you're typing new text and it's indented, you're screwing up and we ask that you fix this before proceeding.