A: What are you doing these day?
B: I'm brushing up my Japanese.
A: Reviewing your Japanese? What for?
B: I'm preparing for the entrance examination of post graduates.
e.g. I shall have to brush up my mathematics before I take on that job.
e.g. He needs to brush up on his English before he goes to the U.S.
e.g. Eleanor spent much of the summer brushing up on her driving.
1. the entrance examination of : ......的入学考试
e.g. I heard you had passed the entrance examination of the university.
2. What for? 为什么？
e.g. Did John work hard at college ? What for ?
E.g. "What for?" echoed the girl.
3. prepare for：为......做准备
e.g. She decided to forgo the party and prepare for the English exam.
e.g. I've been out of circulation in order to prepare for the TOEFL.