The most common point of confusion for learners is the difference between 熄灯 (xīdēng) and 关灯 (guān dēng).
关灯 (guān dēng): This is the neutral, everyday word for “turn off the light.” It means to flip a switch. It is used in 95% of casual situations. You use it when you leave a room, when the sun comes up, etc.
Correct: “我出去一下,帮我关灯。” (I'm stepping out for a bit, help me turn off the light.)
Correct: “天亮了,可以关灯了。” (It's bright out, you can turn off the light now.)
熄灯 (xīdēng): This word carries a sense of finality or adherence to a schedule. Use it for “lights-out” rules or when you are turning the lights off for the very last time before going to sleep.
Incorrect Usage: “我只是去趟洗手间,你帮我熄灯吧。” (I'm just going to the bathroom, help me turn off the light.)
Why it's wrong: This sounds overly dramatic, as if you're going to bed for good. The correct word here is 关灯 (guān dēng) because the action is temporary. Using 熄灯 implies you are not coming back to use the light again soon.
Think of 关灯 as “switching off” and 熄灯 as “extinguishing for the night.”