Qu'est-ce qui or qu'est-ce que?

French often builds questions with a question word + est-ce. The first word tells you what you ask about, the last word tells you its role in the sentence:

• Qu'est-ce qui = what, as the subject: the verb follows directly. Qu'est-ce qui se passe ? (What's happening?)
• Qu'est-ce que = what, as the object: a subject follows. Qu'est-ce que tu fais ? (What are you doing?)
• Qui est-ce qui = who, as the subject. Qui est-ce qui a appelé ? (Who called?)
• Qui est-ce que = who(m), as the object. Qui est-ce que tu attends ? (Who are you waiting for?)

Quick test: does the verb already have its own subject (tu fais, ta mère a dit)? Then use que. Is the question word itself the subject (se passe, t'intéresse)? Then use qui. Before a vowel, que becomes qu': Qu'est-ce qu'il veut ?