What does "que" and "cual" mean without an accent mark? How do they compare when to each other? How do they compare to their accent-marked form?
On spanish.about.com they explain that you would use "que" if you can replace the English "that" with "which" and it would still make sense. So, it's a pronoun. El plan que quiere es caro. (The ...
I've seen the use of interrogative pronouns in ordinary statements, not just questions. For example, from the book "Harry Potter y la piedra filosofal": "Los Dursley se estremecían al pensar qué ...