Bonjour Richard,

La leçon qui répond à votre question est au niveau C1.
La forme est «le ne explétif»
Voici le lien qui associe cette leçon avec «avant que. . .»:

What is the ne explétif and when to use it in French">What is the ne explétif and when to use it in French">What is the ne explétif and when to use it in French">What is the ne explétif and when to use it in French
What is the ne explétif and when to use it in French">Avant que + ne explétif + the subjunctive mood (Le Subjonctif) = Before I do in French

J'espère que ma réponse vous aidera.
Bonne chance et bonne continuation dans vos études en français, la langue de Molière et qui a été utilisée par le monde depuis l’époque d’Hugues Capet

Un the hunt for an example in English, I chanced across a similar construct: "I am not leaving before he has not greeted me." What you want to say is that you won't leave unless he has greeted you first. Of course you can also say "I am not leaving before he has greeted me." but that doesn't sound quite right. The "not" in the example above is also a "ne expletif" just as in French. Just that in French they are more common than in English. -- Chris (not a native speaker).