I found this from another duolingo user: DanielOnye4 4 1 You basically use qu'est-ce qui when "what" is the subject of the question e.g What's happening here? is Qu'est-ce qui est arrivé ici? whereas Qu'est ce que is used when What isn't the subject(which is most of the time) e.g What are you doing? is Qu'est-ce que tu fais?
This is a case of pronominal verbs. When dealing with these verbs, the pronoun (me, te, lui/le/la, nous, vous, les, etc) is placed between the subject (je, tu, il/elle/on, etc) and the conjugated verb. The pronoun is based off who the action is being done to. So in the case of this sentence, "that interests", we can see the word that is the subject because it is doing the action, while the conjugated verb is interests. But we need to ask ourselves "who" the action is being done to. The action in this case is being done to "you". Therefore the correct arrangement in French is "Qui vous intéresse" though in English this reads as "that you interests". Keep in mind that the pronoun "vous" could easily be replaced with "te" (or more correctly, "t' " because of the vowel) if the sentence were informal or talking to a single individual, not a group.
If it's you doing the action, to answer your second question, then the verb is conjugated to match the subject; you already recognized that this is not the case with this sentence, therefore it could not be "You interest what". ;) (Though just for the record, this would be "Vous intéressez quoi" or "Qu'est-ce que vous intéressez")
Some other pronominal usage:
Je t'aime = I love you
He knows us = Il nous sait OR il nous connait
I wake [myself] up = Je me réveille
Sparklehappy -- Thanks. A lot to think about -- I suppose the best practice is to speak and hear so often that my responses and understanding flow from usage, not having to think about it, which obviously wouldn't work in conversation.
As a complete aside, it seems silly when Duo asks: "You’re about to give a lingot away! Are you sure you love this comment so much?"
I've got 370 of the bloody things -- what am I going to do with them anyway?
Other guy gave too long of an explanation so here's mine. 'Qui' is a placeholder and the actual thing that 'intéresser' conjugates from. 'Qui' is used as a dummy subject that the verb acts upon.
That's why the line still holds up if you remove 'vous'. Until the 'vous' is added, it's simply "What is of interest?".
When the subject is "what": "Qu' est-ce qui" Qu' est-ce qui fair du bruit? (What is making a noise?)
When the subject is "who": "Qui est-ce qui" or "qui" Qui est-ce qui parle? or Qui parle? (Who is speaking?)
When the object is "what": "Qu' est-ce que" or "Que" Qu' est-ce que tu fait? or Que fait-tu? (What are you doing?)
When the object is "whom": "Qui est-ce que" or "Qui" Qui est-ce que tu aimes? or Qui aimes-tu? (Whom do you love?)