Questions & Answers about Što je sutra problem?
Why is je right after Što instead of next to problem?
je is a clitic, which is a short, unstressed word. In Croatian, clitics almost always want to be in the second position in a sentence (the Wackernagel position). Since Što is the first word, je must fall right behind it.
Is it possible to say Što sutra je problem?
No, because that breaks the second-position rule for clitics. Što is the first word, so je must immediately follow it. If you put sutra second, je gets pushed to the third position, which makes the sentence grammatically incorrect.
Could I change the word order to Što je problem sutra?
Yes, you can! As long as the clitic je stays in the second position, the rest of the sentence is quite flexible. Što je sutra problem? emphasizes the time (tomorrow), while Što je problem sutra? puts slightly more focus on the problem itself.
What case is the word problem in here?
It is in the nominative case (the base dictionary form). In sentences using the verb to be (like je), the thing you are asking about is always in the nominative case.