^Your explanations are so so helpful!! Thank you for taking the time to help me, Yongbai! 정말 감사합니다!!

2. 내일 저는 오빠랑 같이 공원을 걸을거예요. '조사' is really difficult to learn because it does not exist in English. ‘공원에서’ makes sense but here ‘공원을걷다’ sounds much more natural. 3. 저는 질문을 했지만 아무도 답을 몰랐어요. 제 질문에 답을 아는 사람은 아무도 없었어요. I think you tried to say: I asked a question but no one knows the answer. So better Korea expression would be "저는 질문을 했지만 아무도 (정)답을 몰랐어요." Or "제 질문에 대한 (정)답을 아는 이는 없었어요". 물어보다= 질문하다. 질문=asking a question. You could use “(궁금한점을) 물어보다 or (궁금한점을) 질문하다. But “질문을 물어보다” is an awkward expression. It would be better if you could show what kind of question you have asked in the sentence. for example, 저는 한국의기후에 대해 물었는데/질문했는데 아무도 답을몰랐어요. 4. “직원이” is the right expression. In order to use 조사 properly, one should train his/her ears. It will take time. 5. 창문을 닫아 주세요. 닫다 is the infinitive form. 주세요 is the verb ending with a “polite imperative” meaning. When you combine a verb with the verb-ending 주세요, you should change the infinitive; in this case, 닫다-> 닫아 + 주세요. Other examples: 하다 -> 해주세요.