1. He doesn't arrive at the office until half past nine.
-> It was not until half past nine that he arrived at the office.
-> Not until half past nine does he arrive at the office.
2. I don't know how to ride a bicycle until I was 15.
-> It was not until I was 15 that I knew how to ride a bicycle
-> Not until I was 15 do I know how to ride a bicycle.
CHÚC CẬU HỌC TỐT!!! Cho mình CTLHN nhé!!!