强调句型中,it is .that.不做成分,也就是去掉了不影响句子结构.
比如 it is Harry Potter that is my favourite book.= Harry Potter is my favourite book.
而定语从句中,that是做成分的,代指先行词,如主语,宾语等.做宾语时可以省略.如
Harry Potter is a book (that) I like most.
The book that is my favourite is Harry Potter.that代指主语,不能省略.