1.that的用法。
(1)主语从句、表语从句、同位语从句中用that但不能省略。
That they are good at English is known to us all.
The problem is that we don’t have enough money.
The report that there will be a severe storm in the northern area is false.
(2)一般情况下，宾语从句中的引导词that可省略，但在以下几种情况中that一般不省略：(A)当that从句和主句谓语动词之间有插入语时；(B)有多个that引导的从句时，第一个that可以省略，而其他的that常不可省略；(C)介词except，but，besides，in等后跟that引导的宾语从句时；(D)当when，who，what，where，why，how等引导的从句与that引导的从句作主句谓语动词的并列宾语时。
He judged that，because he was a child，he did not understand wine.
The reason lies in that she works harder than the others do.
Everyone knew what happened and that she was worried.
(3)that和what的区别。
that 引导名词性从句时，在主从句中不担当任何句子成分，也没有任何含义；而what引导名词性从句时，在主从句中都要充当一定的句子成分，what可以分解成定语从句中的“先行词＋关系代词”，即常说的“先行词＋that/which/who”。
It’s a shame that he has made such a mistake.
I will do what I can (do) to help him.
(4)同位语从句与定语从句中that的区别。
同位语从句中的连接词that只起连接作用，在从句中不担当任何句子成分；而定语从句中的关系代词that在句中作宾语或主语，与先行词有修饰关系。如果句子是同位语从句，就应用连接词that而不能用which。同位语从句一般放在表具体含义的名词后解释说明名词的含义或内容，如以下名词：news，fact，suggestion，truth，plan，belief，doubt，possibility，idea等，而定语从句只是对先行词的限定和修饰。
They expressed the hope that they would come to visit China again.(同位语从句)
The hope they expressed is that they would come to visit China again.(定语从句)
2.whether和if的用法。
(1)whether和if在宾语从句中可以互换，但是作介词宾语时连接词一般用whether。
It all depends on whether they will come back.
(2)后面直接跟or not 时用whether。
I didn’t know whether or not he had arrived in Wuhan.
(3)主语从句、表语从句中只能用whether。
Whether the meeting will be put off has not been decided yet.
The question is whether they have so much money.
(4)whether可以引导同位语从句，用以说明前面的名词的内容，if则不能。
We ought to discuss carefully the question whether we can do it or not.
(5)whether常与or连用表示一种选择，if不能这样用；whether也可与动词不定式连用但if不能。
The question of whether they are male or female is not important.
I have not decided whether to go or not.
(6)间接宾语位于句首时或者间接宾语提前时用whether不用if。
Thank you，but whether I’ll be free I’m not sure at the moment.
(7)whether可引导一个让步状语从句表示“不管”、“无论”，而if不能。
Whether he comes or not，we will begin our party on time.
