The book is so helpful that I want to buy it.that的用法?什么成分?

可以的

不正确，要在that之前加一个and就对了 the book is nice and that is what i want（to buy）.

两句话都不正确。 第一句：the book which is nice is i want（to buy） 这句话你用which引导一个定语从句修饰book，这儿没错。错的在后面，the book which is nice is后面连接的是表语从句，缺少一个词来引导这个表语从句，所以要加个what。 the book which is nice is what i want（to buy） 第二句：根据我对第一句的分析，很容易发现这句多了一个连接词that，删去就行了。