You're right, they're antonyms. Using just synonyms makes it too simple, so the rules are: a synonym, an opposite, or an association. So for "before," it can be, for example: ahead, back, after, behind, etc. This creates more variety, but there is still only one answer in the end.
had "thereafter" as solution. I'm not sure how that should work. How is "after" related to "before" other than the negative?