It's all about the order of the text string.
If you open a function (like quote or bold, or anything else) it needs to be closed again to revert to normal, as Josh has said.
This guide explains it in more detail: https://wiki.simplemachines.org/smf/Quote
A good tip is to always use the "preview" button before you post, you will see a preview of how your post will appear, above the text window. I'm pretty obsessive about this if I'm honest and rarely post even the simplest of response without checking the preview