Малчик has no ending vowel, so its genitive case is to add the vowel -а. The vowel -ы is added in the genitive case to words that end in -а or -я.
Furthermore, ы never follows к (and г or х). It is a very important rule of Russian.
I dont get it... the sentence before was У них нет рары, but this one says У мальчика есть мама. Why is the рары genitive and the мама not?
Negation takes genitive. If there's no negation, then it stays in accusitive case.
Мама is the subject of this sentence, so it's in the nominative case, not the accusative. Accusative is used for direct objects. There are no direct objects in the Russian version of this sentence, only in the English translation. The literal translation of the sentence would be something like, " (A) mother exists at (the) boy."
Perhaps because "mama" in English is more a form of address than a descriptive noun like "Mom" = informal "Mother".
In other words, we use "mama" when talking to our mothers ("Mama, can you...."), but refer to them as "Mom" when talking about them. "My Mom is...."
That's not a hard and fast rule, but "My mama is..." is a little odd.
Nitpicking here: "Does the boy have a mom/mum" was accepted but "Does the boy have a mama" was not. All three words (mom/mum/mama) ought to be interchangeable.
What makes this a question and not a statement? Is it just the question mark?
The у is part of a phrase: "y [person/thing] есть [something]" which means literally "by [person(s)/thing(s)] is/exists [thing(s)]". That is transliterated in English into a more idiomatic phrase, "The [person(s)/thing(s)] has the [thing(s)]"
From a source I don't have a cite for:
Genitive case is used to show that something (somebody) belongs or refers to something (somebody)...<pre>
берег реки the bank of the river</pre>
It depends on the question, so:
Do you have water? У тебя есть вода? (to have example)
He is eating. Он ест (to eat example)
Do you want to eat? Ты хочешь есть? (to eat example)
I'm starting to wonder how russians can tell a question from a statement, seeing as they're both said the same way.
How would I know if it is a question or an affirmation without reading the sentence?
So the only thing in audible speech that would make this a question is the infection. The word order it's correct for it to be a question as well as a statement?
Ударение вопросительное в предложении не стоит на слове "есть". Предложение звучит как утверждение, а не вопрос. The sentence sounds like a statement, not like a question!