My French teacher recently told me that the interrogative qui can be used both as an interrogate as well as taking the place of a pronoun. I may be incorrectly interpreting what she said and this may not actually be the two usages of qui. I just remember that she said it functioned differently than other interrogatives; why is this?
Qui IS a pronoun : It can play the same role as a noun in a sentence.
It can be use for interrogating or as a pronom relatif that describes a noun. It is used to replace a person (and sometimes an animal too). It is thus similar to "who"(or sometimes the old "whom"). In a question, "qui" is the unknown part of the sentence. You expect that the answer will replace "qui" with a noun (e.g. the name of the person who is knocking on the door.
Qui frappe à la porte ?
La personne qui frappe à la porte semble pressée.
Note that qui is here the subject of frapper, but it can also play other roles in the sentence e.g.
A qui parlais-tu ? Qui as-tu invité à ton anniversaire ?
However, as a pronom relatif, il will no more be used as a complément d'objet direct. You must use que instead.
La personne quE tu as invitée à ton anniversaire m'a confirmé sa venue.