Hi Vincent,
Here is my take.
(A) In this time, the book store’s manager brought us to his office.
(A) At this time, the book store's manager brought us to his office.
(B) The sale was around the corner. So all the market even the book store had a big sale.
(B) The sale was round the corner. So all the market including the book store had a big sale.
(C) She wanted to buy books because her exam are / were coming soon.
(C) She wanted to buy books because her exams were coming soon.
(D) There was a girl who had fainted. Susan asked people to move her up to the manager office. She realized the girl was her classmate – / , Lee.
(D) There was a girl who had fainted. Susan asked people to move her up to the manager office. She realized that the girl was her classmate.
(E) While I reached there, I saw many book stalls / book booths selling books.
(E) While I reached there, I saw many book stalls selling books.
(F) She heard a commotion from the crowd. She was calm because she knew how to use first aid to help people.
(F) She heard a commotion from the crowd. She remained calm because she knew how to use first-aid to help people.