Third, reviewing and previewing are necessary.

第三，复习和预习是必要的。

DCS plays a major role in previewing the document.

DCS对于预览文档起重要的作用。

The teacher's assignment includes previewing Unit 5.

教师的作业包括预习第5单元。

To buy some useful reference books when previewing...

买些有用的参考书在预习时看。

We should get into the habit of previewing and reviewing.

我们应该养成预习和复习的习惯。