that作为表语从句的引导词在该表语从句中不充当句子成分， 不能省略)// That is what I want to tell you.那就是我想要对你讲的。
“That is why...”与“That is the reason why...”同义， 只不过从语法结构上讲， “That is the reason why...”中why引导的是—个定语从句， 将其中的the reason去掉则与“That is why...”结构一样， 例如：That is (the reason) why I cannot agree. 这就是我不能同意的理由。
“That is because...”句型中从属连词because引导的名词性从句在此作表语， 这也是个常用句型， 意为“这就是为什么……/因为……”。“That is because...”与“That is why...”之间的不同在于“That is because...”指原因或理由， “That is why...”则指由于各种原因所造成的后果