"Kto wyłączył światło?"

Translation: Who turned the light off?

[lunatix17]

Doctor Who fans, anyone ?
"Hey ! Who turned off the lights !"

[albanaich]

How do we know whether the light is on or off?

[Jellei]

off = wy-

on = w-

Yeah, they're very problematic in pronunciation, sometimes you really need to stress the beginning of the word.

[Walkinthedog]

Shut off the light is also a common term used in the states.

[Jellei]

OK, added now.

[Steven895145]

Also you can turn the light out.

[Jellei]

A new thing learned... added.

[mark.boats]

Is it a general rule to use the male form of the verb when asking a question about who has done something?

Would that also apply to declaratory statements like "Ktoś wyłączył światło" - "Someone turned off the light"?

[Jellei]

Yes, both "kto" and "ktoś" are always grammatically masculine (singular), even if you're a female teacher in a girls-only school.

[mark.boats]

Ah OK, I guess it follows logically that "nikt" would also be masculine, ie: "Nikt nie wyłączył światła" - "Nobody turned off the light"...?

How about co and coś? My gut feeling is that they would remain neutral but Polish is full of surprises... :D

[Jellei]

Yes, "nikt" as well.

Your gut is correct, "co" and "coś" (and "nic") are neuter singular :)