In the example in your original post, I don't think it's to do with cases, as «куда» is an entirely different question word from «где». «Где» = where, but «куда» = to where. If you know the archaic English words "whither" and "whence", it will make this distinction easier...

«куда?» «туда.» ("whither?" "thither.")
«где?» «там.» ("where?" "there.")
«откуда?» «оттуда.» ("whence?" "thence.")

I think that asking the correct question could be somewhat useful to non-native speakers, even in the case of cases (no pun intended)... just remember what each question really means. For example, «чего» = of what, «кому» = to whom, «чем» = with what, etc.