A: I don't know why you change your mind. You are looking forward to traveling abroad, aren't you?
B: Yeah. But I must buckle down on my study this holiday. If I fail the entrance examination, my parents will be very disappointed.
A: I see.
Buckle down的意思是“开始认真干”，其英文解释为：To put forth maximum effort toward something, especially after not having done so.表示“认真，全力以赴做某事”，在短语的后面的用介词on，即：buckle down on something.
e.g. If you want to get a passing grade this semester, you'll really need to buckle down and study hard.
e.g. The only way we'll meet the schedule is to buckle down and get on with the job.
e.g. Johnny found he'd better buckle down on getting himself a job.
1. change one's mind：改变主意
e.g. I hope you wouldn't chop and change your mind so often.
e.g. She' ll work on him to make him change his mind.
2. look forward to：期待，期盼，渴望
e.g. I look forward to your letters almost madly, and your handwriting makes my heart beat as if I were a schoolgirl.
e.g. We appreciate your cooperation and look forward to receiving your order early.
3. fail the examination：没有通过考试
e.g. Do something about your English, or you will fail the college entrance examination.