that前面可以加逗号吗
不可以，that只能引导限定性定语从句，也就是不加逗号的。当然如果有插入语会有逗号。加逗号，也就是非限定性定于从句，只能用which,whose,who等等,不用that.
that的用法
1、that 作指示代词
如：That is the greatest guarantee of success.
那是成功的最好保证。
2、that 作指示形容词
如："Do you remember that guy you were dating? What happened to him?" she asked.
你记得和你约会的那个小伙吗？他发生什么了？她问。
3、that作关系代词引导定语从句
如：This is the sort of thing that Jeremy loves.
这是杰里米喜欢的东西。
4、that 作连词
that 可以作连词引导各种名词性从句，包括宾语从句、主语从句、同位语从句和表语从句。
1) that 作连词引导宾语从句
They have recommended that the site be closed and repaired.
他们建议将该处关闭并进行维修。
2) that 作连词引导主语从句
That you will win the medal seems unlikely.
你想获得奖牌看起来是不可能的。
3) that 作连词引导同位语从句
The idea that modern art can only be seen in museums is mistaken.
现代艺术只能在博物馆才能见到的想法是错误的。
4) that 作连词引导表语从句
One problem is that I don't look any different from other people.
有一个问题是我看起来和其它人没有任何不同。
5、that作副词
that和 this 都可以作副词，用于修饰形容词或修饰另一个副词，表示“这么，那么，如此，这般”的意思，如：
There aren't that many people here.
这儿没这么多人。
6、that的常见句型搭配
that还有很多词组和搭配，尤其需要熟悉下边这些与英语句型有关的结构。
1）so that以便, 以使；以致
如：Read the advertisements so that you can compare prices.
你应该看一些广告以便比较价格。
2）in order that为了
如：I am saving money in order that I can buy a house.
我正在攒钱，以便能买一所房子。
3）except that除了(...之外); 只是
如：Act 2 is similar, except that the orange block is smaller.
第2幕(跟第1幕)类似，除了那个橙色块小了一些。
