如果我是个结巴，大家说可以加进几个that？

I think that1 the2 XX3 (that4 the5 student wrote on the blackboard) was wrong.

哪来的that which? 这里真没有。

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关于题主一直在问的 that which，这个有。

把that which看成一个较古旧、或很正式的结构就可以了

它相当于what，语法上大概是那么回事。

A very small part of acting is that which takes place on the stage!

That which is most highly valued in the tribe is valor.

前一个that是指示代词，后一个which, 自然是关系代词。

 这两句的划线部分都应改一下，如分别为 sth that, The thing that.

###  "当先行词是that的时候，关系代词不能再用that, 而应选用which."

这个很难说对与不对，因为that which现在用的本来就不多，1940年后，基本没再回升。