Why do you use Ctrl+Enter to move down a line? Are you used to sites and apps that require using Ctrl+Enter and Shift+Enter to do that?
You don’t need to do either on the forum. Ctrl+Enter is the quick key to submit a post. Enter is how you make a new line (just like a text document). If you’re on laptop, just hit Enter and nothing else to move down a line.
I’ll look into seeing if there’s a way to let users change their keyboard shortcuts, but I think this is just resulting from a habit that doesn’t apply to the site’s text box.