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重点讲解：
1. used to do sth. 过去常常；过去曾；
I used to play here as a boy.
我小时候常在这里玩。
2. take refuge 庇护；避难；
They took refuge in a bomb shelter.
他们在防空洞中避难。
3. be aware of 注意到的；察觉到的；意识到的；
Advanced students need to be aware of the importance of collocation.
层次较高的学生必须意识到词语搭配的重要性。
4. again and again 再三；反复；
The same changes took place again and again.
同样的变化一再发生。
n. 小路，跑道，踪迹，轨道，乐曲
v. 跟踪
