I think the difference depends on if there is a subordinate sentence following the expression "antes de".
If you say "antes de que" (+ subj), you need a new sentence: Antes de que... (tú vengas / nosotros compráramos el piso / yo me haya instalado...).
If you say "antes de", you must go on with a noun: Antes de... la hora fijada / del accidente. Of course you can use a sustantivated (?) verb that is, a verb which works as a noun (infinitive): antes de empezar...
Please, tell me if there is sth you can't understand or not properly expressed.
