1. It was not until 30 years old that she became a teacher
2. If I had had her address , I would have seen her
3. Not until she was 20 years old, did she become a doctor
( Câu 3 có lẽ đề bài bạn viết thiếu động từ ở vế sau của câu , mình thêm động từ thành "was" nhé)