Do you mean "what" instead of "where"? It's not incorrect to say "where", but it would mean you were asking about the location of the problem. If a verb is before its subject, the sentence is a question. Ex. -What is the problem? -How are you? If the word order is normal, then the sentence isn't a question (or it's made a question by another part of the sentence). You can think of "what the problem is" as a noun. Ex. -How do I found out what the problem is? (the question is formed by "how do I", but the rest of the sentence remains unchanged) -I don't know what the problem is. -Tell me how you are. "What the problem is" is incorrect on its own because it can't be a question and it's not a full sentence. It could be used as part of a bigger sentence though. "How do I find what is the problem?" is incorrect because "what is the problem" isn't making the sentence a question; it's just acting as a noun in the sentence.

B and C are correct. A and D are wrong. In questions, you invert the order of subject and verb (if there is an auxiliary verb like do, be, have, modals, you invert the auxiliary verb, not the main verb). Verb then subject e.g. Do you have some time? Have you any time? Can I help you? . In statements, the order is usually subject then verb. e.g. I have time. I can help you. The confusion in your examples arises from the fact that question words such as "how", "why" can also be used in statements. e.g.I know how to help you. I don't know where he is.

u know sometimes sth mustn't have any substantial difference ,they don't have any difference in terms of meaning but the first one is more formal and beautiful than the second ,i wish i think right!