"après" and "avant" with de and nouns

Question

I have found four sentences in the French grammar book I have just read that all show different combinations of de after après and avant.

1. Nous partons tout de suite après le cours
2. Elle se brosse les dents après manger
3. Nous dinons avant de regarder le film
4. C'est un petit plat à manger avant le diner

What are the rules around de and le with avant and apres...my guess is the following

Before an infinitive, de is shown after avant but is not shown after après. Before a noun, no de is ever shown

Is there a reason why de is shown after avant but not after après?

Answer 1

Before verbs, this question would be a duplicate as mentioned in the comments. (usually "avant de" + infinitif présent; après + infinitif passé)

Before nouns, "avant" and "après" are directly followed by a "noun group". This noun group will be composed of "déterminant + nom" like in other situations (like the start of the sentences). There are plenty of "déterminants" that you can use and one of them is "de" (not the preposition in this case, but a contraction of "des" before an "adjectif épithète"). Sentences with "après de" are therefore quite rare, but possible.

Il est parti après de longues heures.

De longues heures se sont écoulées avant qu'il ne parte.