Final answer:
The rule for using the preposition 'por' in the sentence 'Juan se siente mal por su error' is 'because of,' 'on account of,' 'for' when referring to cause or reason.
Step-by-step explanation:
The correct rule for using the preposition por in the sentence 'Juan se siente mal por su error' is 'because of,' 'on account of,' 'for' when referring to cause or reason. In this sentence, the preposition 'por' is indicating the reason or cause for Juan feeling bad - his mistake. It is used to express the cause or reason behind a feeling or action.
Learn more about Prepositions in Spanish