Breakdown of Свет в столовой горит, но кто-то гасил его ночью.
он
he / it (for a masculine noun) / it (for a neuter noun)
в
in / to (direction)
свет
light
ночью
at night
Questions & Answers about Свет в столовой горит, но кто-то гасил его ночью.
Why use the imperfective past гасил here when the action of turning off the light was clearly completed?
This is a classic example of the "annulled result." The person successfully turned off the light, but the result of that action no longer exists because the light is back on right now, as established by горит. If you used the perfective here (like погасил), it would imply the result is still active and the light is currently off.
Are горит and гасил the standard words for electric lights?
Yes! While you might already know включать/включить (to turn on) and выключать/выключить (to turn off) for electronics, Russians frequently use гореть (to burn) to say a light is currently on, and гасить/погасить (to extinguish) to say it was turned off.
Why is the word for "night" (ночь) written as ночью here?
To say when an action happens during a general time of day or a season, Russian uses the instrumental case. The nominative ночь becomes the instrumental ночью to mean "at night" or "during the night."
Could I use кто-нибудь instead of кто-то for "someone" in this sentence?
Not in this context. You use кто-то when the "someone" is a real person who actually performed an action, even if their specific identity is unknown to you. Кто-нибудь is used in hypothetical situations, questions, or requests where you mean "anyone at all."