在带有分词的句子中，该如何判断分词部分是是谓语还是作为表语？
如：
He is doing something.
通常来讲是理解为“主谓宾”结构的进行时态，但能不能理解为这是一个“主系表”结构？即is是系动词，doing something为ing分词做表语。
She is looking after the children. （主语 + 谓语动词 + 宾语）
Her job is looking after the children. （主语 +系动词 + 表语）
动名词作表语和进行时态有什么区别
“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.和Her job is looking after the children. 它们太相似了，只不过主语不同，其他成分完全一样，意思却大相径庭，这是为何？
这确实是一个非常有意思的问题。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。由于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维持不变：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的目前分词构成进行时态，所以is前后两部分不可以互换。不可说：*Looking after the children is she. 又如：
His hobby is collecting stamps. 他的业余喜好是集邮。
→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. 不能说：B is A. （is doing意为“正在做”）
动名词作表语：
His hobby is collecting stamps. 他的业余喜好是集邮。
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. 治疗你的病的唯一方法是上床睡觉。
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. 这个男生的兴趣就是玩网上游戏。
My favourite is playing tennis. 我最喜欢的是打网球。
Her first delight was going to the Tower. 她第一个想做的事情是参观伦敦塔。
【结构和意义特点】
这种结构的实质是：be 是系动词，be 前的主语和 be 后的表语是同位定义，动名词是主语的内容和讲解。主语总是表示“工作，目的，目的，想法，愿望，喜好，工作，责任，办法”之类的名词。
