"A girl is eating an apple."
Translation:Девочка ест яблоко.
Both may be acceptable depending on context, but generally девочка is the direct translation for girl. Девушка is a woman starting from the teenage years and can refer to adult women who are not obviously old. I would say that 'young woman' or 'young lady' is the proper translation for девушка, not girl.
The most common example I can think of is anonymous interactions—if you want to speak to a random young woman on the street in English, you might use "ma'am" (especially in the US), in Russian you would use девушка.
Great question! Russians only capitalize proper nouns (Санкт-Петербург) or the initial word of a sentance: Вы были в Санкт-Петербурге? (See https://everydayrussianlanguage.com/en/russian-grammar/russian-grammar-rules/)
"ест" means "is eating" (e.g "Он ест" = "He is eating). "есть", with a soft sign ("ь") indicating palatalization, means "there is" or "there are" (e.g "В меиро есть человек" = "There are people in the subway"). There is also "У меня/тебя/вас/наго/неё/них есть", which is equivalent to "I/you/we/he/she/they have/has". Do you understand?
I think it would have been better to say "the little girl" and not only the girl. Girl also refers to a young person... The same problem as in German "Mädchen". Besides, there are here futher exercises in which the two nouns are accepted. The Russians are more accurate.
I think "A girl is eating apples": Девочка ест яблоки. You will often see "и", "a", "я" or "ы" ending for plural. It's a bit more complicated in Russian, there are exceptions even beyond these; I think when speaking of Rubles or hours they even have an ending for singular, one for a few (2-4) and a third for many (5 & above.) Perhaps that's why when I first asked about plural endings, the Native Speaker I was asking just laughed! ( :