writing exercise, "The town of Gruyères", grammatical point

Question

In the exercise "The Town of Gruyères", the translation of "Before we even entered the picturesque village," is given as "Avant même que nous entrions dans le bourg/village pittoresque,". I think it should be 'nous sommes entrés dans ...'. What am I getting wrong there? Also, I'm aking my self, does an optional 'ne explétif' go before 'entrions'?

Accepted Answer

Sydney,
 ‘avant que’ requires the verb following to be in subjunctive form, as used in the exercise. This is not imparfait, although the conjugation of entrer with nous is the same in both imparfait and subjunctive.
The ‘ choice ‘ here is between present subjunctive and past subjunctive. 
Present subjunctive is most commonly used, and here it is indicated because the timeline is that this event (entrer) is to occur after the visit to the cheese factory, not before.
 https://progress.lawlessfrench.com/revision/grammar/how-to-use-avant-que-ne-expletif-subjonctif-before-i-do 
https://progress.lawlessfrench.com/revision/glossary/conjugation-tense-mood-aspect/french-subjunctive-past-tense-le-subjonctif-passe 
( NB at the moment the 2 links above show the same website name when posted, but they link to different topics on avant que and past subjunctive respectively)
A  ‘ ne explétif ‘ can be added, usually only in writing nowadays, but even in writing it is optional and formal. There is no ‘ negative idea ‘ here either to lend more to its use.
An opinion from Grevisse himself, according to Bescherelle, is interesting :
Maurice Grevisse se réjouit de la suppression progressive de cette particule inutile qu’il qualifie de parasite. 
https://bescherelle.ca/le-ne-expletif/

Answer

Thank you Maarten for your prompt, helpful reply