在中学英语课本中，动名词和不定式作主语的句子较为常见，但它们的含义有所不同。现在将动名词和不定型式作主语的含义、位置及习惯用法分述如下：
一、动名词和不定式作主语的含义
一般来说，动名词作主语表示一种抽象的和泛指的动作，而不定式作主语则表示一次性的、具体的动作。换句话说，动名词作主语通常不与特定的动作执行者联系在一起，而不定式作主语往往与特定的动作执行者联系在一起。例如：
Lying is wrong.（泛指lying的做法或作风）
To lie is wrong.（对于说谎者是谁，说话人心中是有所指的）
To live in Beijing is the height of Tom’s ambition.（对某个特定的动作执行者而言）
Swimming is good exercise.（指游泳这项运动）
二、动名词和不定式作主语的位置
1．动名词作主语通常有两种位置，一种是把动名词直接置于主语的位置。例如：
Saying is easier than doing.
Seeing is believing.
Crossing the Atlantis（大西洋）by plane takes only a few hours.
第二种是用it作形式主语，把作真正主语的动名词置于句末。这通常用于某些形容词或名词之后。例如：
It’s interesting planning a holiday.
It’s rather tiring walking around in a city.
It’s fun having guests for the weekend.
We don’t think it’s a bore doing the same kind of work ev ery day.
动名词作主语的上述两种位置有时可以互换，但第一种形式比第二种形式的泛指意义更强些。例如：Lining in Beijing must be wonderful
It must be wonderful living in Beijing
2．不定式作主语通常也有两种位置，与动名词作主语时的位置相同，即第一种是把不定式作直接置于句首担任主语。例如：
To learn English is not an easy thing.
To be here is a great pleasure.
To remember to cut off the electricity is important.
To hesitate is a pity.
不定式作主语的这种形式一般用得不多。现代英语倾向于用it作形式主语，把不定式置于句末。例如：
It’s a pity to hesitate.
It took（us ）five hours to get here.
It made me happy to find Mary there.
It needed hard work to finish the job.
注意：
（1）如果主语和表语都是不定式，则只能采用第一种形式。例如：
To be kind to the enemy is to be cruel to the people.
To see is to believe.
（2）如果是疑问句或感叹句，则只能采用第二种形式。例如：
What is it like to be there？
How long did it take you to finish the work？
What a joy it was to read Barbara’s bo ok！
三、动名词和不定式作主语的习惯用法
虽然动名词和不定式都可以作主语，但以下几种情况时，人们习惯用动名词，而不用不定式。
1．主语与表语一致，即当表语是动名词时，主语也要用动名词；当表语是不定式时，主语也要用不定式。
Teaching is learning.（= To teach is to learn）
Seeing is believing
2．在“There BE+no+主语”这种结构中，通常用动名词作主语，而不用不定式。这种结构表示“不可能、无法”，相当于“It is impossible to do sth”。例如：
There is no getting along with him.（=It is impossible to get along with him.）
There is no joking about it.
注意：本句型中的no有时可用not any或never any代替。例如：
The man is a bore，but there is not any getting him away.
There is never any telling what will happen in the future.
3．当use，good，waste，bore，fun，shame，encouragement，misery等名词作表语时，通常用it作形式主语，把作为真正主语的动名词后置。例如：
It’s no use talking to him again.
It’s no good（your）refusing to do it.
It is a waste of time trying to reason with him.
It’s a bore doing the same kind of work every day.
It’s fun having guests for the weekend.
It’s such an encouragement being able to pass all the exams.
注意：“It’s no use /good+动名词”这种结构可改成“There BE+no+use /good+动名词”结构。例如：
It’s no use talking to him again .= There is no use talking to him again.
It’s no good（your）refusing to do it .= There is no good（your）refusing to do it.4．当，interesting，tiring，terrible，nice，better，foolish，enjoyable，worthwhile，useless，dangerous等形容词作表语时，通常也用it作形式主语，把作为真正主语的动名词后置，例如：
It is dangerous risking climbing a tree.
It’s terrible not being allowed to smoke at all.
It’s interesting planning a holiday.
It’s nice talking with you，John.
注意：important，necessary，possible，impossible，essential等形容词不适用于上述结构。例如，不能说：
It’s important learning foreign languages.
应改为：It’s importa nt to learn foreign languages.
5．在疑问句中，多用动名词的复合结构做主语，而不用不定式的复合结构。例如：
Isn’t our saying so right？
Isn’t his leaving like that strange？下载本文
