Da ich nichts zu tun habe, gehe ich ins Fitnessstudio.
Weil ich nichts zu tun habe,gehe ich ins Fitness.
Which one is more commonly used? Da or weil? And which one is dialectal?
The sentence with weil is definitely more commonly used in the spoken language. It has not much to do with dialect; I would say both versions are fine throughout Germany. One point is that when we speak, we often say it the other way around
If you want to sound really native-wrong German, then you have to ignore the rule of putting the verb at the end in the relative clause. Therefore, on the street you would probably hear