I'm confused about the difference in using "avant que" et "quand".

Bonjour Teresa ! There are two distinct expressions in French, and therefore don't behave in the same manner. "Avant que" expresses an action taking place *after* the main clause: Le Subjonctif implies there's a doubt as to whether this action will take place, as it hasn't happened yet. Whereas "quand" expresses a simultaneity, hence it being more assured and therefore followed by L'Indicatif mood. Other expressions behaving like "quand" would be: "lorsque" (when), pendant que (while), une fois que (once), etc... I hope that's helpful!