Translation:There is still a little milk in the bottle.
When does one acknowledges the "Il restes" means "There remains" and not take the more literal "He leaves". My main confusion arrives from taking "Il" as "He" but not sure when that doesn't applies. For example, "Il faut" it's clear (so far at least) that translates to "It's needed". What's the rule here?
You have the sense of it but the English is not correct. Rester here is more in the meaning of "to be left" (although it also means to stay, to remain), so Il reste can be rendered as "There is left..." There are some good examples of this use here: http://www.larousse.com/en/dictionaries/french-english/rester/68014
I think that what you propose would be translated as, "Un petit peu de lait reste dans la bouteille."
In the sentence provided by Duolingo, they use an "expletive construction" (there is) using the impersonal "il." (Ask yourself: what does the "il" stand for in this context after all? Here we can see that this "il" is impersonal.)
Think of the impersonal 'il' in 'il y a.' If I were to say, "Il n'y a pas de temps à perdre", I would translate that as There is no time to lose, and not "Time is not to lose."
On top of that, your construction could even be used as a command. Imagine telling someone, "A little bit of milk stays/[must stay/must remain] in the bottle." That would be, "Un peu de lait doit rester dans la bouteille."
The reason I asked why my construction wasn't accepted was because Duo does accept the construction "A little bit of milk REMAINS in the bottle", which is a one word difference from my suggestion. If "rester" can mean either "to stay" or "to remain", and Duo accepts the construction listed above, then it should also accept the one I presented.
I see. For the sake of translation, I can see why they would accept "A little bit of milk remains in the bottle", because it sort of still captures the spirit of the French sentence, and I suppose that's what translating is about. However, in this context, I think there is a subtle nuance between "remain" and "stay" here, because "stay" seems like it can only be used when the subject of the sentence is something that can move independently unless we are talking about what should be done is or usually done. (The book must "stay" on the shelf/The book stays here.)
Inversely, if you were to ask a French-speaker to translate, "There is a bit of milk left in the bottle", would that not be different than asking them to translate "A bit of milk stays in the bottle" ?
"Stay" and "remain" can probably both mean "rester" in certain contexts, but after some time, you'll be able to discern when one should be used over the other.
I was thinking about the differences and this is what I have:
- Ma mère va rester à la maison. → My mother is going to remain/stay at the house. ↓
rester here can translate to remain/stay.
However . . .
- Il ne me reste plus qu'un an (à l'école/au lycée, etc.) → [loosely:] I only have one year left of school/There's only one year left of school for me. / There remains for me only one year of school. ↓
Stay would not be an appropriate translation in this particular instance.
All of that said, I understand your frustration. Perhaps Duolingo will eventually accept it anyway, and I'm not sure how I'd feel about that. Power to the people, though.