Not sure I can discern the crux of the difference between antequam and ante (According to L&S (II.B.1.a vs II.B.2), ante might be used with verbs, like antequam). In other words, in what occasions we are bound to use only one of them? — when they are not simply interchangeable.
The word ante is both a preposition and an adverb, whereas antequam is only an adverb — or more accurately antequam falls under the adverbial use types of ante and antequam is essentially a conjunction. If you need a preposition, ante is your only option. If you need an adverb, the difference between the two is small.
In L&S both words are given under ante, with antequam only appearing in part II in adverbial usage, mostly in II.B.2.
I will let the details of this difference be fleshed out in possible other answers, because I just wanted to give this one point.