"When is it correct to use these words?" "I don't understand the difference between them very well, or which one is grammatically correct." They're both correct and have slightly different meanings—although I would put "to me" at the beginning of the sentence. "For me" is used to talk about something's effect on you. "To me" is used to talk about your opinion. In your sentences, the difference is very subtle and the sentences are more or less the same. The test was really hard for me = I found it difficult. To me, the test was really hard = I think it was difficult.

Hi David, The test was really hard for me > Para mí el examen estuvo dificil. (correct) "for me" expresses your opinion "to me" is more like "a mi" and wouldn't make sense unless you said something like "The test SEEMED really hard to me" Other examples: The tacos were too spicy for me. Today was too hot for me. She spoke really fast for me. (Here, "to me" would just be a fact that she was speaking to you quickly, and "for me" means that you had a hard time understanding)

Hi David, I am a native English speaker learning French. Sentence 1 is correct, sentence 2 does not make sense in fact.The test is hard FOR a person, not to. Try to fit the sentence in a category below: To: Expresses direction/ transfer towards, ex: She walked to the door; He gave some money to charity. Expresses purpose (verb + to + verb), ex: We ran to catch the train Shows a time period, ex: He works from 9 a.m. to 5 p.m. Indicates a limit, ex: The cup was full to the top For: Expresses advantage/disadvantage of a person/ thing, ex: He opened the door for me. This is the category your sentence comes in. Expresses reason (because of), ex: He runs for his health Shows the duration of time, ex: He works for 8 hours a day Indicates a distance ex: He ran for 6 miles Keep learning. You are doing great! :)

Both are correct but the semantics are slightly different. To me - implies the state, as presented to you, the test was very difficult, even if you had studied. For me - implies the experience. Your experience with the test was difficult. The test is easy but since you didn't study, it was difficult.