What is the most important action to take when starting to taxi an aircraft?

The most important action to take when starting to taxi an aircraft is to check the operation of the brakes. This action is crucial because the brakes are vital for controlling the aircraft's movement while on the ground. Ensuring that the brakes function correctly helps prevent potential accidents during taxiing, where the aircraft needs precise control to navigate safely.

While other actions, such as communicating with ground control and ensuring passenger safety, are also important, they follow after confirming the aircraft is capable of stopping and controlling its movement. Without proper brakes, the pilot cannot effectively manage the aircraft on the taxiway or during takeoff procedures, greatly increasing the risk of incidents that could lead to injuries or damage. Therefore, verifying brake operation is paramount and should be prioritized before any other ground operations.