Questions & Answers about Здесь нельзя выключать свет.
Why does the translation say "You must not" when there is no word for "you" in the Russian sentence?
This is an impersonal sentence, which Russian uses a lot for general rules. Words like нельзя (forbidden/must not) and можно (allowed/may) don't use a standard subject like "I" or "you". Because there is no specific person mentioned (like вам нельзя - for you it is forbidden), it applies to everyone in general: "One must not" or "You [anyone] must not."
How do we know нельзя means "must not" here instead of "it is impossible"?
The meaning of нельзя changes depending on the type of verb it is paired with. When paired with an imperfective verb like выключать, it means an action is forbidden ("must not"). If it were paired with a perfective verb (выключить), it would mean the action is physically impossible ("cannot").
Why do we use the imperfective verb выключать here instead of the perfective выключить?
In Russian, general prohibitions (telling someone not to do something as a broad rule) almost always use the imperfective aspect. Нельзя выключать means it is a general, ongoing rule not to turn off the light.
What case is the word свет in this sentence?
It is in the accusative case because it is the direct object receiving the action of being switched off. Because свет (light) is an inanimate masculine noun, its accusative form looks exactly the same as its dictionary (nominative) form.