“动名词作表语”和“进行时态”形似而神异。请看这两个句子：

She is looking after the children.

Her job is looking after the children. 

它们太相似了，只是主语不同，其他成分表面看起来完全一样，意思却大相径庭，这是为什么？

这确实是一个很有意思的问题。Her job is looking after the children. 意为 “她的工作是照料孩子的。” 本句中，is 是系动词，looking after the children是动名词作表语。因为is是系动词，主语和表语是平等的并列关系，所以，主语和表语可以互换而意思保持不变：Looking after the children is her job. 照料孩子是她的工作。

She is looking after the children. 意为“她正在照料孩子。”本句中，is是助动词，跟动词look的现在分词构成进行时态，所以is前后两部分不能互换。不可说：*Looking after the children is she. 又如：

His hobby is collecting stamps. 他的业余爱好是集邮。

→Collecting stamps is his hobby.

He is collecting stamps. 他正在集邮。

不可说：* Collecting stamps is he.

His ambition is working as a lawyer. 他的抱负是当律师。

→Working as a lawyer is his ambition.

He is working as a lawyer. 他正在以律师的身份工作着。

不可说：*Working as a lawyer is he.

【规律】A代表“主语”，B代表“动词-ing形式”：

“主系表结构”：A is B. 可以说：B is A. （is意为“是”）

“进行时态”：A is B. 不可以说：B is A. （is doing意为“正在做”）

动名词作表语的例句：

His hobby is collecting stamps. 他的业余爱好是集邮。

His ambition is working as a lawyer. 他的抱负是当律师。

The only way to cure your disease is going to bed. 治疗你的病的唯一办法是上床睡觉。

Her job is keeping the offices clean. 她的工作是给办公室保洁。

Her work is teaching children music. 她的工作是教孩子音乐。

His only wish is finding a job in an office. 他唯一的愿望就是找一份办公室的工作。

The ant queen’s full-time job is laying eggs. 蚁后的专职工作是产卵。

The boy’s interest is playing games online. 这个男孩的兴趣就是玩网上游戏。

My favourite sport is playing tennis. 我最喜欢的运动是打网球。

Her first delight was going to the Tower. 她第一个想做的事情是参观伦敦塔。

现在进行时态的例句：

The scientist is doing experiments no one has so far attempted. 科学家正在作一项迄今为止没人做过的实验。

According to these figures, our company is doing well. 从这些数字来看，我们的公司经营得不错。

The teacher is reading Tom’s English paper. 老师正在审阅汤姆的英语答卷。

He always makes careful notes while he is reading. 当他阅读时，他总是仔细作笔记。

The witness was telling about the whole accident. 目击者正在讲述整个事故的过程。

He is working in his photographic studio. 他正在摄影室工作。

The sculptor is working on a sculpture of Venus.  那个雕刻家正在雕一个维纳斯雕像。