这个好象不是很复杂的.
教你个办法：
不定式、动名词、还有that从句做表语的功能是：解释说明主语内容,一般可以与主语互换位置.
比如你的句子,可换成：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“将来”或"be+V-ing"表示进行进,则不能换了：
如：I am to leave soon. 就不能与主语换位了.
She is looking after the children.