1.定义：在系动词后面也可跟一个有连接词引导的句子；
2.系动词：分为be动词，后面能接形容词的感官动词及相当于be动词用法的实义动词。
①be动词：am/is/are/was/were等；
②后面接形容词的感官动词：feel/sound/look/taste/smell等；
③相当于be动词用法的实义动词：become/keep/remain等。
e.g.Tom is a good doctor.
They are friendly.
That sounds great.
We should keep quiet in the classroom.
3.引导词：
(1)定义：连接主句和从句的词；
(2)分类：连词，疑问代词，疑问副词；
连词：只起连接作用，不作成分，包括that(无意思), if(是否)和whether(是否)；
疑问代词：在从句中作主语，宾语或定语等成分；有what (事情或样子), which,
who, whose等；
疑问副词：在从句中作状语，有where,
when, why, how等；
疑问词：疑问代词和疑问副词统称为疑问词，关系词在从句中作成分。
4. that引导的表语从句:“主语+系动词+ that从句”；that不作成分，只起连接主句和从句的作用；在表语从句中不可省略，称作连词。
e.g. The fact is that he didn’t even take the exam.
The trouble is that I have lost the document.
5. wh-疑问词引导的表语从句：wh-疑问词引导的从句作“This is”和“That is”的表语时，它们通常不含有疑问意义，而是分别表示具体的地点，时间，原因，方式等。其中what(事情或样子), which, who, whose等为疑问代词，作主语，宾语或定语等成分；where(地点), when(时间), why(原因), how(方式)等为疑问副词，作状语。
e.g. That is why he was late for an hour.
This is where the great man once lived.
She is no longer what she was five years ago.
What is troubling us is where we can find him.
6.从属连词as if, as though, because, just as等引导的表语从句:
e.g. It looks as if we will be late.
He looked just as he had looked ten years ago.
7. 注意：
(1)主句的主语是reason时，表语从句要用that，不能用because;
e.g. The reason(why) he was absent was that he was ill.
The reason why he was dismissed was that he was careless and irresponsible.
(2)引导表语从句只能用whether，不能用if。
e.g. His first question was whether Tom had arrived yet.
8.专题练习：
(1) The trouble is _______ we are short of money.
(2) That is _______ stone walls are used instead of fences around New England fields.
(3) At that time, ______seemed as if I couldn’t think of the right word anyhow.
(4) The trouble is ________ he lost his money.
(5) The question is _________ we need more ice cream.
(6) The problem was _______ it was too valuable for everyday use.
(7) What she couldn’t understand was _________ fewer and fewer students showed interest in her lessons.
答案见于公众号初高中英语学习资料
