It's OK to discuss if in English "my sister" is better than "the sister" or "a sister". But that comes up only when translating the sentence into English. English tends to use the possessive pronoun instead of articles, for instance the Spanish "él chocó El carro", usually is translated "he crashed HIS car" instead of "her crashed THE car". Of course in English you can say the car, his car, get the, this car, that car, when needed. But it's very common to say his or her car.
But the possessive tends to be used in English where other languages don't use it. So the possessive may have originated in English when translating from Russian to English.
"Is eating" sounds more natural in English for something that is being done right now, rather than a repeated occurrence. If she's eating one apple today, then another tomorrow, and another the day after that, then she eats my apples. If you're just saying that she's eating one right now, using "eats" is grammatically correct, but normally you would say she is eating my apple.
I'd guess "Sis" as a translation of сестра wasn't accepted. You could report that, but I would recommend translating as sister in future.
It is because яблоко is a neuter word.
мои is a plural.
мой [moy] is masculine.
моя [moya] is feminine.
моё [moyo] is neuter.
Credit to garpike and ILoveHens ( https://forum.duolingo.com/comment/15030386/What-is-the-difference-between-%D0%BC%D0%BE%D0%B9-%D0%BC%D0%BE%D1%8F-and-%D0%BC%D0%BE%D1%91 )
@ericashelton - Because there's no article or anything, that's definitely a legit translation, but "a sister" in English sounds like a female member of a religious order (like a nun at a convent).
With such limited context, both "a" and "the" sound kind of weird to me though :\
This is simply incorrect. I wrote: "A sister is eating my apple" and it was marked wrong and should be "The sister is eating my apple.". I'm just reviewing basics, and I'm telling you ... it's not right to mark that as wrong. "A sister" is just as acceptable as "The Sister". Doesn't matter how it could be used. With how we have to insert the indefinite article in English for the lack thereof in Russian? Both should be correct.
From what I've gathered about Russian, just thought I'd share this to help some confusion:
Сестра usually would refer to one's own sister, моя is implied and not needed ('my sister' is accepted as part of the answer). Russians would not assume you are talking about a nun or someone else's sister.
Because of word ordering (which you will slowly understand better throughout the course), сестра appears first, suggesting that we are talking about a particular sister, not just any sister in general. Russian doesn't use articles, but often used word order to convey that information. This is probably why 'the sister' is accepted but 'my sister' is not.
The past tense of yest is yel/yela/yelo - that would be imperfective (she was eating my apple). The perfective would be "съесть". Past tense would be Съел/Съелa/Съелo which would mean she ate the apple and the action is done. Sorry for switching from Latin to Cyrillic there...
яблока is genetive, so you would use it if it's a single apple but genetive, i.e. negation (у меня нет яблока I do not have an apple), possession (the apple's X, X of the apple, the apple has X), as well as after certain prepositions that require genetive (возле яблока near the apple).
Edit: removed suggestion that яблока is plural - this is incorrect. See below.