- What is eating the girls? = Что ест де́вочек? (this is usually used non-metaphorically, i.e. some monster is literally eating them; if you want to ask why they are sad, you could say Почему́ де́вочки грустя́т? 'Why are the girls sad?')
- What are the girls eating? = Что едя́т де́вочки?
The difference is:
- Едят is a plural verb form, ест is a singular verb form. When что is a subject, you need to use the singular verb form. When де́вочки is a subject, you need to use the plural verb form.
- Девочки is a nominative case (used for the subject), де́вочек is accusative (used for the object).
No. Что is used with singular verb forms, and «едя́т» is a plural form, so you can’t translate it this way. Also, «де́вочки» is in the Nominative case, which is used for the subject of the sentence. To make it an object, you would have to use the Accusative case: «де́вочек».
So, "What is eating the girls?" would be translated «Что ест де́вочек?». (Creepy!)
Only creepy if you take it literally. "What's eating them?" is an idiomatic way of asking what is causing their sad mood.
EDIT: That being said, idioms can often be lost in translation when translated literally, so perhaps the better question is how would the same sentiment be expressed in Russian?
Oh, that would probably not be expressed with "ест". "Что их снеда́ет?", with a older and more poetic word "снеда́ть", will work in that meaning, but not "ест".
(Or if you don't need the eating metaphor, you could simply ask "Почему́ они́ грустя́т?" or "Почему́ они́ гру́стные?" 'Why are they sad?'.)
If I've understood comments in other threads correctly that would shift the emphasis of the question from the girls to what they were eating would it not? In English such an emphasis would usually be used to express disgust, or disbelief at the nature of what was being eaten. Would such a thing translate like that? (or am I completely off track?)
You don't. Что 'what?' is grammatically singular, but it can refer to either one or several entities. (The same is true for кто 'who?').
Actually, this is same as in English: you say 'who eats the rice' and never 'who eat the rice', because who and what can't take plural verb forms. However, 'who eats rice?' doesn't imply there's only one person eating the rice; so it's grammatically singular, but can refer to one or several entities.
The English grammar comment is right with a few caveats (at least that I can come up with at the moment). The biggest one is that this rule applies when we're talking about questions. When "who" is the subject of a subordinate clause, it can take a plural verb: "People who study Russian are well-advised to spend the time to master the cases."
And for questions, there's an exception when the verb is a form of "to be" linking the question word to a plural noun phrase:
- What are your favorite Russian dishes?
- Who are some famous Russian chefs?
This is an interesting question. "What are girls eating?" sounds odd by itself, but "What are girls eating nowadays?" for example is ok and asks about all girls in general. The present progressive without an additional time adverb makes it a specific question, implicitly about a certain bunch of girls, so the definite article corresponds. In simple present, "What do girls eat?" is completely fine and, again, is about all girls.