The thing is it's a writing thing, or a formal way of speaking. If the lesson goal is to reproduce native like way of speaking, they shouldn't use it. Nobody says that in everyday conversations, only if they are giving a formal presentation.
May 15 at 12:31
It's the so-called "expletive ne". As explained above it doesn't add to the meaning and is only used in formal contexts, not in conversation. Let me add that you may encounter it not only after "avant que" but also in other contexts, for example after verbs expressing fear:
Je crains qu'il ne vienne : I fear that he comes (may come)
In some comparisons:
Il y avait moins de monde qu'on n'avait pensé: There were fewer people than was anticipated
And a couple more cases
It's a historical phenomenon based on the (outmoded) idea that some sentences are "kind of" negative. It ultimately comes from the Latin syntax
In all those cases, notice something: there's a "ne" but there's no "pas" or other negative words (rien, personne, ...) after the verb, pointing to the fact that these are not really negative sentences.
May 15 at 18:17
thank you. I thought I asked a stupid question. it's actually quite interesting.