主语是句子的主体，是一个句子中需要加以说明或描述的对象。主语一般由名词、代词或数词充当。
e.g. The plane has just taken off at the airport.
e.g. They are students from the United States.
e.g. Twelve divided by three is four.
另外，动词非谓语形式和名词性从句都可以充当句子的主语。
e.g. To get up early is good for our health.
e.g. What they said sounds good.
2、谓语
谓语通常是动词，位于主语之后。
e.g. The man dived deep in the sea.
谓语动词不只限于单个动词，也可以带有助动词。
e.g. We will have an examination next week.
e.g. He has been away from home for months.
句子必须有谓语动词。当英语和汉语表达习惯有差异时，不可遗漏谓语动词。而且其他词类不能充当谓语动词。
3、宾语
宾语是及物动词所表示的动作的对象，或是介词所表示的某种联系的对象。换而言之，及物动词和介词后都要带宾语。宾语一般由名词和代词充当。
e.g. The waiter brought two cups of coffees to the table.
e.g. The lady was looking at herself in the mirror.
e.g. He seemed to be searching for something.
动词非谓语形式和名词性从句都可以充当动词和介词宾语。
e.g. We hope to get some help from them.
e.g.She hates troubling her classmates.
e.g. The book is about how pandas live in the mountains.
4、定语
定语是对名词或代词加以修饰或限定。定语通常由形容词担任，也可以由副词、介词词组、非谓语形式充当。
e.g. There is a long line waiting to get the new stamps.
e.g. I’m sorry I don’t know the man over there.
e.g. I wonder whom the books on the shelf belong to.
5、状语
状语主要用以修饰动词、形容词和副词，一般由副词充当。
e.g. The problem is pretty hard.
e.g. The guests enjoyed the Chinese dishes very much.
介词词组、非谓语形式和从句也可以充当状语。
e.g. They managed to finish the job before Monday.
e.g. The old man sat on the beach, enjoying the afternoon sun.
e.g. As he was leaving the office, it started to rain.
6、补语
补语补充说明主语或宾语的身份或特征。位于连系动词后的补语也称为表语(predicative)，一般由名词或形容词担任。
e.g. Adam is a tailor.
e.g. Things got worse and worse.
句子成分除以上6种主要成分外，还有同位语(appositive)和独立成分等次要成分。
名词或代词后面跟有其他名词或词组，对前者作补充说明，称为同位语。名词性从句也可以作同位语。
e.g. The Great Wall, one of the wonders in the world, attracts a large number of foreign friends.
e.g. The news that he had invented a new type of car delighted us.
三、句子基本类型
句子的基本类型
句子是由不同的句子成分按一定的语言规则相互组合而成。不同的句子成分在句中有序排列，形成不同的句型结构。英语有7个基本句型。
下列字母代表各句子成分：
S-主语 V-谓语 动词 O-宾语 C-补语 A-状语
下面，老师选了一些2020各区一模阅读首字母部分的句子，你能试着用所学的句子成分来分析一下吗？
But I was a__81__ that people would feel bad for me if I lost.
（解析：通过分析句子中的动词was可知，后面缺的是表语；所以必须填形容词；答案为afraid）
Since I was c__83__ prepared, I felt that my chances of winning were strong.
（解析：通过理解从句中的since可知，这是原因状语从句；然后再分析从句的句子成分可知，主语为I，系动词was，表语为prepared，所以所填入的成分为修饰表语的状语（副词）， 因此答案为completely）
