It's quirks like this that remind me my Russian isn't as good as I think it is. I'm very used to reading in Russian, and in a book, article, or blog post, context would tell me this is singular.
Looking at isolated sentences however, I actually have to remember non-intuitive plurals!
I got this one right only because I thought:
Hang on a minute, the girls have everything, but I thought that was the nominative plural, yet it must be genitive because у... есть; is the genitive plural the same as the nominative plural?
Then hovered over the word to see, and behold, genitive singular looks like what one would expect the nominative plural to look like if, like me, one pays the bare minimum of attention to grammar lessons in favour of leaping into reading / listening / etc, hoping to pick up the finer points of grammar as one goes along.
Ironically, as DL famously "doesn't teach much grammar", I find it most useful for exactly that, because it'll highlight my errors, so I learn as I go.
Genitive case is used to associate a noun with an "owner". In English, we might associate genitive case with the phrase "of the ...". For example, in "The book of the song." The phrase "of the song" would be replaced by the genitive case of the noun for "song" in a language with cases. In the phrase "song book", the genitive case of the noun for song would modify this to clearly denote that it means " the book of the song" instead of " the song of the book", for example.
Genitive is a form of the word that implies ownership. In English the noun doesn't change - you just say "I have an apple". But in some languages the verb changes form to show it belongs to someone. Think of it like a word changing its ending so it became "Here apple-mine" or "He apple-his". By using the genitive form, you know the noun is associated with the subject. Might seem a bit unnecessary but then a lot of English does to Russians. Why do we have to say "the" and "a" all the time they might ask! ;)
Why is the translation in this example девочка -> девочки rather than девочка -> девочкы?
I've looked at some other Russian language learning sites which say the ending of a feminine noun in the genitive case, such as девочка, removes the -a ending and replaces is with -ы. On that same note, if the feminine noun ended in -ь, then the ending for the genitive case would be changed to -и.
So... is the translation incorrect, are these other sites incorrect, or am I misunderstanding the translation entirely?
I am not sure why this old comment got -2, maybe because of the link without embedded explanation. So I will copy the rule from the link here:
Spelling Rule #1 After the velar consonants г, к, and х: and the sibilant consonants ж, ч, ш, щ: one must never write the "hard" vowel ы, but must always replace it with its "soft" equivalent и, even though after ж and ш, и is pronounced as if it were written ы.
Всё is singular (neuter) and все is plural. If you happen to know some spanish,it is just like the difference between "todo" and "todos". "Toda" would be вся. So всё usually means the "all of it" "the whole", while все is like "everybody" or "each and every one of" or "every". "Всё вино" is all the wine, while "все вина" is all the wines.
Well, in common written Russian, yes both words can be written as все because it is common to to not write ё.
But they are different words and are always pronounced totally different in modern Russian speech, so while learning, it is better to avoid confusing them and understand that все and всё are not the same and if you pronounce one when you should pronounce the other one, it will be wrong.
У девочек есть всё.
You can see all the cases of a word in wiktionary. https://en.m.wiktionary.org/wiki/%D0%B4%D0%B5%D0%B2%D0%BE%D1%87%D0%BA%D0%B0