动名词一般是动词+ing使其作为名词使用而具有动词的意思，所以它就能作为主语了，比如Hard working is the way to success.而主语从句是指用一个完整的句子做为一个新句子的主语，比如：That you are playing with children is fun.在这句话中，you are playing with children 就是主语，但是这样会显得主语过长，有头重脚轻的感觉，一般写成It is fun that you are playing with children，使用it作为形式主语，you are playing with children是真正主语。其作为宾语情况和主语相似。...
