不定式作表语
① My work is one thing.
② I must clean the room every day.
合并
My work is that I must clean the room every day.（转化）↓
My work is that I have to clean the room every day.（must = have to）
简化
My work is to clean the room every day.我的工作就是每天打扫房间。
简化过程
My work is that I have to clean the room every day.
①去掉that，that无实际意义！↓
My work is I have to clean the room every day.
②去掉I，I 与主句的my意思重复了！简化就是要去掉那些重复的和无关紧要的！↓
My work is have to clean the room every day.
③去掉have，因为人们为了最简形式，将have的部分意思赋予了to，此时的to ，不是介词to。此时的to clean就是 have to clean，也就是must clean!↓
My work is to clean the room every day. 我的工作就是每天去打扫房间。
动名词作表语
① Her job is one thing.
② She looks after the children.
合并
Her job is that she looks after the children.
简化
Her job is that she looks after the children.
简化过程
Her job is that she looks after the children.
①去掉that，that无实际意义！↓
Her job is she looks after the children.
②去掉she，因为she与主句her意思上重复了！简化就是要省去重复和无关紧要的！↓
Her job is looks after the children.
③此时的句子↑是错句！两个动词不能联用！所以只能将looks转变词性！↓
词性：looks（动词）→looking（动名词）
④此时的句子符合语法↓，而且具有名词性质的表语从句 that she looks after the children 简化成了动名词短语 looking after the children。词性和意思都没有多大变化，是一种极好的简化！
Her job is looking after the children.
意义：简化的目的是为了提高交流的效率。
有趣的事
Her job is looking after the children ＝ Looking after the children is her job.
为什么会这样，下回说！
