请问That/It is the first time (that) I have read the book.这句话为

这个句子后面的(that) I have read the book.修辞的是the first time为定语,所以为定语从句
主要是看句子修辞的那个先行词代表句子的哪个部分,修辞的是主语部分,就是主语从句,修辞的是定语部分,就是定语从句.
再问： 要是能举个例子就更好了 谢谢
再答： That he finished writing the composition in such a short time surprised us all. That he finished writing the composition in such a short time 这前半部分就是主语从句，因为整个句子可以看做是“That he finished writing the composition in such a short time”(主语） "surprised"（谓语） "us all"（宾语） 所以前面这段就是“that"引导的主语从句 给你几个主语从句的例子： Whether we will go for an outing tomorrow remains unknown. Who will be our monitor hasn't been decided yet. Whom we must study for is a question of great importance. What caused the accident remains unknown. Who the watch belongs to is unknown. He is the man （whom/that） I saw yesterday.这里的（whom/that） I saw yesterday.就是定语从句，起到限定修饰”the man"的作用 给你几个定语从句的例子： The man whom you spoke to just now is our English teacher. The prosperity which/that had never appeared before took on in the countryside. A child whose parents are dead is called an orphan He lives in a room whose window faces south. By the time when you arrived in London, we had stayed there for two weeks.