A tenant association (Mieterverein or Mieterschutzverein) gives legal assistance to renters. When you are a member of a Mieterverein, you can get help from their lawyers. They can check your lease, help you with a bad landlord, get your rent reduced, and help you sublet an apartment.
After you join a Mieterverein, you must wait a few months before you use their legal services.1
The Berliner Mieterverein and Mieterschutzbund Berlin are the two biggest associations in Berlin. The membership costs 50€ to 100€ per year.