What must you remember when coupling up a trailer?

Your answer is correct. When coupling a trailer in Germany, the first step is to connect the service line (yellow hose coupler) before the emergency line. However, if the hose couplers are combined into a single unit, you connect both lines at the same time to ensure proper operation of the trailer's braking system. This procedure is important for safety, preventing brake failure or unintended trailer movement. The emergency line (red hose coupler) is not connected first because it serves as a safety backup and needs the service line to be properly connected first. Remembering this sequence ensures that the trailer is safely and correctly hooked up.