It isn't wrong, but it adds a nuance that isn't in the Swedish sentence, so it won't be an accepted answer.
You need do in English to create negated sentences (I read -> I do not read) and questions where there isn't a question word: (Do you read?). In those cases, do does not change meaning. But if you add do here, it gives a meaning close to 'actually', 'in fact' or something like that.
I can only guess from German that "vad djur" wouldn't be grammatically correct. "Vad" is "what" as in "what's that" or "what are you doing". But it can't be used to ask "what (kind of) book/person/animal". In these cases you have to use "vilket/n" instead. However I think (!) you could ask "Vad för djur äter ost?". But I can't guarantee that this is correct.
English is unusual among European languages in allowing both "which" and "what" to be used by themselves as interrogative adjectives directly before a noun:
1. Which animal eats cheese?
2. What animal eats cheese?
Most languages use a translation of "which" rather than "what" in these questions, or else a construction like "what for (a)" = "vad för (en)"
In English there is a slight difference between 1 and 2. 1 assumes that there is a specific list of possibilities that both the persons asking and answering are aware of. 2 makes no such assumption:
"What tie will you wear tomorrow?"
"You own several neckties. Which tie will you wear tomorrow?"
If it's vilket, then you know it's singular.
Vilket = which (singular ett word).
Vilken = which (singular en word).
Vilka = which (plural).
Can this be correct "Vilket äter djur ost?". We put question first, then comes the verb and rest comes after. I read this in the tips. I get confused when some times that rule is followed and sometimes not. Anyone can kindly tell, if above version is wrong, how to know for sure the order of 'question word' 'verb' 'noun/rest of words' ? Thanks
Your suggestion does not work. It is like saying in English "Which eats animal cheese?"
In a question of the type here, the first word must be a question word, or a noun phrase that begins with a question word. For example:
1. who eats cheese?
2. what eats cheese?
3. what animal eats cheese?
4. which animal eats cheese?
In the noun phrase, you cannot split the question word (which/vilken/etc.) from the noun that follows it.
I have used examples in English, but Swedish follows the same pattern.