What should you do when you see this road sign?

You answered correctly.

This road sign in Germany indicates a mandatory left turn. According to the rules, you must signal your intention to turn left in good time (indicate left) to inform other road users. Then, you should make the turn before reaching the sign itself, not after it. This means you prepare to turn and execute the turn before passing the sign.

Turning left behind the sign is incorrect because the sign marks the point at which you must have already started your left turn.

Remember, this type of mandatory sign always requires you to follow the direction indicated immediately at or before the sign's position, and signaling is essential for safety and legality.