Lights and indicators
Lights are how the vehicle speaks. Headlights for seeing and being seen, dipped for oncoming traffic; indicators for intention; brake lights for slowing; and the reversing light, which comes on with reverse gear to tell everyone behind what you are about to do.
Check them all regularly, including the ones you cannot see from the driver's seat. A failed brake light or indicator removes the warning other drivers rely on and is an offence.
Signal before you act, not as you act, and cancel the indicator afterwards. An indicator left flashing tells the wrong story to everyone who sees it.