Newbie Can't Make Variables Work

Sorry for this. I feel like a fool. I've been using Twine for less than a few weeks and I've primarily used it for interactive stories, but now I'm trying to do different kinds of games and I feel really silly.
I don't understand the rules of variables?
I know the whole thing from the neocities guide:

(if: $legs is 8)[You're a spider!] will show the You're a spider! hook if $legs is 8. Otherwise, it is not run.

But I can't understand how to phrase else-if and things like that. Sorry if this isn't clear enough. Can someone just... Hold my hand and explain plainly how to use a dang variable. How to use else-if and either and the other things.