How do I make an “else” in an IE HTML conditional?

You’re not looking for an else, you’re looking for <![if !IE]> <something or other> <!--[endif]> (note that this is not a comment).

<!--[if IE]>
   You're using IE!
<![endif]-->
<![if !IE]>
   You're using something else!
<![endif]>

You can find documentation on the conditional comments here.