這個(gè)好象不是很復(fù)雜的.
教你個(gè)辦法：
不定式、動(dòng)名詞、還有that從句做表語的功能是：解釋說明主語內(nèi)容,一般可以與主語互換位置.
比如你的句子,可換成：To teach them English is my job.
又如：
My idea is that we will start tomorrow.
=That we will start tomorrow is my idea.
Her job is looking after the children.
Looking after the children is her job.
但是,"be+不定式"表示“將來”或"be+V-ing"表示進(jìn)行進(jìn),則不能換了：
如：I am to leave soon. 就不能與主語換位了.
She is looking after the children.