that当作为连词时怎么使用？

1.引导宾语从句（可以省略）。

  例：

I think (that)he is very good.

2.引导定语从句，代替其前面的先行词。(作主语时，不能省略；作宾语时，可以省略）

  例：

This is the teacher that/who gave us a lesson yesterday.

I got a present (that) he bought in this shop.