What is the right course of action here?

In this situation, you have a yield sign, so you must give way to other vehicles. The yellow van is going straight ahead, and the red car is turning left. According to German traffic rules, vehicles going straight generally have the right of way over turning vehicles unless otherwise indicated.

Your planned action is to turn left, so you must let the yellow van pass first. You do not have the right to go before the yellow vehicle because it is coming from the opposite direction and going straight.

The red car turning left is your direct opponent, but because it is turning left from the opposite side, you both must yield to the straight-going yellow van. However, between you and the red car, you have the right of way first because you are turning left and the red car is also turning left from the opposite direction, so you cross paths.

In summary, you must allow the yellow vehicle to pass first. That is why the correct action is to let the yellow vehicle go before you complete your turn. This rule ensures smooth traffic flow and prevents collisions at intersections controlled by yield signs.