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 plan that/which he wants is expensive.)
You would use "de que" in any other case. It's then a conjunction.
El plan de que los estudiantes participen en las actividades es caro. (The plan that/
whichstudents participate in the activities is expensive.)
In another post on this site, the most upvoted answers says:
You need to replace "que" and everything after it with "eso" (which means "it"), then you will easily see which one is wrong.
I've seen a couple of examples, however, where I was confused that they sometimes used "que" and sometimes "de que" although I can't see that the above rule would be applicable.
I only have one example present right now, which is taken from spanishdict.com:
No es culpa mía que estés enamorada de mi.
Applying the first rule above, the proper translation into English would be "that" as a conjunction. Using "which" is just wrong.
It's not my fault that/
whichyou're in love with me.
According to that first rule, "de que" would be correct and, hence, the example given were wrong.
Considering the second rule, there's already an "it" in the sentence. Attaching another "it" would be just wrong anyway:
*No es culpa mía (de) eso.
*It's not my fault (of) it.
Eso no es culpa mía.
It is not my fault.
So, if I applied that second rule rightly, the use of "que" were correct.
That raises the following questions:
- Which of the two rules cannot applied to this example? Which rule is 'incomplete', that is? (If I'm not mistaken, the first rules dismisses "que" as a conjunction altogether.)
- Assuming that the "less good rule" is still generally true, how would you extend the rule to cover that case?
- What's correct in this very sentence anyway?