（一）句子成分的定义：构成句子的各个部分叫做句子成分。句子成分有主要成分和次要成分；主要成分有主语和谓语；次要成分有表语、宾语、定语、状语、补足语和同位语。
（二）主语：主语是一个句子所叙述的主体，一般位于句首。但在there be结构、疑问句（当主语不是疑问词时）和倒装句中，主语位于谓语、助动词或情态动词后面。主语可由名词、代词、数词、不定式、动名词、名词化的形容词和主语从句等表示。例如：
During the 1990s, American country music has become mo。re and more popular.（名词）
We often speak English in class.（代词）
One-third of the students in this class are girls.（数词）
To swim in the river is a great pleasure.（不定式）
Smoking does harm to the health.（动名词）
The rich should help the poor.（名词化的形容词）
When we are going to have an English test has not been decided.（主语从句）
It is necessary to master a foreign language.（it作形式主语，真正的主语为后面的不定式）