主语
主语是一个句子所叙述的主体，一般位于句首,通常由名词性的词来充当。可由1.名词、2.代词、3.数词、4.名词化的形容词（如the rich）5.不定式、6.动名词、7.短语、8.从句等表示。

1.名词作主语
During the 1990s, American country music has become more and more popular.

2.代词作主语
We often speak English in class.

3.数词作主语
One-third of the students in this class are girls.

4.不定式作主语
To swim in the river is a great pleasure.
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）

5.动名词作主语
Smoking does harm to the health.

6.名词化的形容词作主语
The rich should help the poor.

7.短语作主语
How to do well is an important question.
Early to bed and early to rise makes a man healthy.

8.从句作主语
When we are going to have an English test has not been decided.
That he isn’t at home is not true. (that不能省略)