Both "Why are you sad?" and "Why you are sad?" are grammatically correct, but they are used in different contexts.
"Why are you sad?" - This is the more common and standard way of asking the question. It follows the standard English word order for questions, which is to start with the question word "why" followed by the auxiliary verb "are" and then the subject "you."
"Why you are sad?" - This is an example of a question in which the subject and verb order is inverted. It is a common construction in some other languages, but it is not the standard word order in English for questions. However, this word order can be used in certain situations for emphasis or to convey a more informal or casual tone.
In most cases, you would use "Why are you sad?" because it adheres to the standard English word order for questions. It's clear, unambiguous, and easily understood by native English speakers. On the other hand, "Why you are sad?" may sound less natural and is typically used in specific contexts, such as in informal speech or when trying to mimic a particular style of speech or dialect.
So, to sum up, "Why are you sad?" is the more appropriate and commonly used form in standard English for asking the question.