So the point is if they used to OFTEN meet there, we use WENN. meaning they used to meet each other in Museumquartier when ever they had the chance. On the other hand, if they met once (or almost once) in PAST in Museum Quartuer, we want to use ALS in this sentence. In fact the freuqency they met helps us to chose between ALS or WENN.
The last topic is about “währendâ€. Remember we can use it as “Temporal Konnektorâ€ as well as “Temporal Präpositionâ€. The difference is when you use it as preposition, you need to use “Genitivâ€ form after it:
Ich fliege nach Sardinien, während meines Urlaubs.
Ich fliege nach Sardinien, während ich Urlaub habe.
We all know about “Nebensatz” and “Hauptsatz“. “Hauptsatz” (“Independent claus” in English, FAN BOYS!) is the one with the verb in the second position in the sentece structure. “hauptsatz” is a sentence that makes sense even without the “Nebensatz”. However “Nebensatz” (“Dependent clause” in English) is the one with the verb (if it has any) at the last position and its meaning is dependent to the “Hauptsatz”.
Exercise: try to mark the “Nebensatzâ€ and “Hauptsatzâ€ in the sentence “Ich fliege nach Sardinien, während ich Urlaub habe”. What happens to the position of verb versus subject if we swap the place of NS and HS?