As Carl said, it's a question of grammar. In that question, there is one QUE and one QUI. You can imagine that the QUI is the subject of the verb "va". It means, "WHAT is wrong?" (Qu'est-ce qui ne va pas?) There is also a QUE in that question, which means "WHAT IS THE THING that is wrong?" Here are some other examples. You can have two QUE: Qu'est-ce que tu fais? (what are you doing?) (que = object of "tu fais") You can have two QUI: Qui est-ce qui parle? (who is speaking?) (qui = subject of "parle") One QUI, one QUE: Qui est-ce que tu regardes? (what are you looking at?) (que = object of "regardes") I hope this helps! You can also see another explanation here: http://www.columbia.edu/~fms5/fint.html

Bonjour Feng, "Qu'est ce qui" is the subject in an interrogatory sentence, equivalent to "what" in English. More specifically, it is equivalent to starting a question with "what" in English, when the subject is a thing. For example: What is happening? = Qu'est ce qui se passe? What is behind that book? = Qu'est ce qui se trouve derrière ce ce livre? What is on the menu? = Qu'est ce qu'il y a sur le menu?