Wenqing, I give it a lot of thought if I should pursue this argument further, as it is not that important to me. But I am really pissed at your native speaker, not you, for giving you such lousy consultancy.
Let’s fix his English grammar and understanding and give him a bit of "accent".
Question 1. Understandably vs. Understandingly · “So, I smiled understandably and remained silent.” It means “I” was an ignorant social awkward, and it is quite understandable that "I" only smiled and did not say anything, because I did not have a clue. · “So, I smiled understandingly and remained silent.” It means “I” was an ignorant social awkward, and that was why “I” only smiled to show that "I" understood what he said. (Grammatically acceptable, but he had no clue!) · I told you that both were fine. But you seemed to believe that “understandingly” was more grammatically correct. How is your choice more “accent”-free? Question 2. Let’s get over this “the” thing once for all. When editing someone’s work, you don’t touch things in the quotation marks. But that is not my point here. You did not give me your reasons, now, I will give you mine and I would like to hear comments from your “native speaker”. First, let’s go back to “the” basics. · With “the” option. “Excuse me, but, is this the dentist’s office?” It means the man was asking if he was in HIS dentist’s office, not in some other dentist’s office. Grammatically correct, but the addition of “the” was meaningless. Why would the man care about if he was in the right dentist’ office? Aren’t you twisting my story to your understanding? How does that remove my “accent”? · With “a” option. “Excuse me, but, is this a dentist’s office?” It means the man was asking if he was in a dentist’s office or a madhouse. Grammatically correct, and I think “a” is a much more logical option than “the”. · Without articles. “Excuse me, but, is this dentist’s office?” It means the man did not care if the office in which he was staying was a specific one, because it is not relevant and important in the context. He was in a panic and would bolt at slightest confirmation that he was at a wrong place. · Why is “the” option “accent”-free? Isn't it a writer's choice? Question 3. What is considered a good writing? We leave it. You keep your "the" and I will keep my GRE words.