Learn Russian: У тебя есть вопрос?

Russian rarely uses a direct verb for 'to have'. Instead, we use a special phrase that literally translates to 'by you there is...' or 'at you exists...'. The preposition у means 'by' or 'at', and есть means 'is' or 'exists'.

Why use тебя instead of ты?

The preposition у always forces the next word into the Genitive case. Тебя is the Genitive form of the informal 'you' (ты).

If тебя isn't the subject, what case is вопрос in?

Вопрос (question) is in the Nominative case (the basic dictionary form) because it is the actual grammatical subject of the sentence. The sentence literally means 'A question exists by you'.

How do we know this is a question when there are no question words like 'do'?

In Russian, you don't need helper words or a different word order to make a yes/no question. The statement 'You have a question.' and the question 'Do you have a question?' look exactly the same in writing. When speaking, you just raise your voice pitch on the word you are asking about.