看你把that放在什么地方.
1.作指示代词,同this用法.
This/That is an apple.
2.作形容词,同this用法.
I prefer that/this one.
3.做连词,引导从句时,同which（指物）
I will buy the red one that/which my wife likes very much.
4.做连词,在强调句中,同who（指人的时候）
It was me that/who locked the door.
5.作副词表示程度时,近so:
I like it so much.
I really care about you that much.
6.作形式主语,近it：
That was my pen.
It was my pen.
以上种种,有的是用法一致,意义有别.有的可以互换,也有特殊情况不能互换的.这里不多说了.
