If replacing "that" with "this" yields a valid English sentence, then you mean "este/esto/esta." Otherwise, you mean "que"Example: "That man is stupid." --> "this man is stupid" --> use "este""He said that I should." --> "*He said this I should" --> use "que"
This one is actually quite straightforward. Other times, it's trickier, like when I try to teach the difference between the past tense and the past participle by making use of the fact that English makes this distinction for irregular verbs only:
Replace the verb in question with a form of "write." If you say "wrote", you need to conjugate a past tense verb. If you say "written", you need the past participle.
"I played baseball" --> "I wrote baseball" --> past tense
"Baseball is played all over Cuba" --> "Baseball is written all over Cuba" --> past participleHere, the trouble is that semantically, "write" (or whatever irregular you choose to use) may or may not make any sense in the sentence in question, thus confusing the student.
Nonetheless, it is my belief and experience that transformations of these kinds of easy for students, whereas recognizing and labeling parts of speech is difficult (probably because the parts of speech are usually taught using incorrect and confusing generalizations). That said, the question remains: how useful is it to have labels for the categories? Is it better to teach "if you can replace that with this, it's a demonstrative" or to teach "if you can replace that with this, use 'este'"?
More provokingly, is there a way to teach such that students will never make the este/que error in the first place. The parts of speech are so far apart that it seems very difficult to imagine they should ever be linked in a student's mind, except through English... it is intriguing to imagine an immersion approach that might eliminate the step through English on the way to learning these structures and thus eliminate the error altogether. Nonetheless, it seems quite possible to me that even if the teacher never said to students that "que" means "that", they would still figure this out and thus be vulnerable to this sort of error...