宾语从句分为三类:
(1) that引导的宾语从句
l hope (that) it will snow this winter.

(2) whether/if引导的宾语从句I'm not sure if he will come.

(3)疑问词引导的宾语从句
I want to know what he will do next.

当谓语动词表示肯定的概念,如“希望、相信、知道、说”时，其后面的句子一般用that引导。that没有任何词汇含义,只有语法功能,目的是使读者清楚后面的句子是宾语从句。多数情况下that可以省略。例如:l hope (that) it will snow this winter.
Betty thinks (that) trees can improve the air.l know (that) he is friendly.

注意:并非所有的动词后面都可接that引导的宾语从句，常接that宾语从句的谓语动词有:believe, expect, explain, feel,hear， hope, imagine, prefer, promise,report，say, see, tell, think, under, stand,warn, wish 等。