"Since when is it dark?"
Translation:Il fait noir depuis quand ?
If you get the French first: "Depuis quand fait-il sombre ?" you would translate to "since when is it dark (or has it been dark)?" or "how long has it been dark?" and it only relates to daylight in French, because of verb "faire" (il fait froid, il fait chaud, il fait sombre...).
But if you get the English first, "since when is it dark?" you can translate about daylight or about any animal or inanimate object - except daylight- that "it" would represent: "depuis quand est-il sombre ?" - Note the change of verb, then, from "faire" to "être".
It would be "dès qu'il est sombre".
In this case we meet gor the first time a french construction that differs from its English equivalent. This construction is "il fait", which litteral meaning is "he/it does/makes", but in this construction translates into "it is".
Il fait, whith the meaning It is, is very common. Now we have to learn in wich cases to use it!
I am not a native English or French speaker, but the sentence definitely sounds the question of the daylight, and "Il fait noir depuis quand ?" means to me, he makes black since when ? Anyway, this question is not appropriate for me, from both side E<>F, at least this question is not for beginners.