What is prohibited when using motorways?

You answered correctly. On German motorways, stopping on the hard shoulder is prohibited except in emergencies, because it can be dangerous and obstruct rescue vehicles. Reversing is forbidden due to high speeds and traffic flow, which would cause severe hazards. Performing U-turns is also not allowed, as the motorway is designed for continuous forward traffic, and such maneuvers could lead to serious accidents. These rules ensure safety and smooth traffic flow on motorways.