一种情况是ing或to do作主语至于句首时：
1)动名词作主语一般表示比较抽象的经常性的行为.例如:Smoking is forbidden in the cinema.抽烟在电影院是禁止的.
2)不定式作主语一般表示具体的某次动作,特别是将来的动作.例如:To know the theory is one thing ;to put it into practice is another.懂得理论是一回事,将它付诸实践又是另一回事.不定式作主语时,常用形式主语it,将不定式后置; 动名词作主语,一般不用形式主语it,但有些词作表语时（如；no use ,no good ,a waste of time/energy ）一定要用形式主语.如；
It‘s not easy to be a good teacher .当一名好老师是不容易的.
It‘s no good talking a lot without doing anything .光说不做是没有好处的.
有时候二者可以互换.Seeing is believing =To see is to believe
还有的情况是非谓语动词至于句首作修饰成分.
作独立成分To tell you the truth,I don't like the way he talked.
当句子是祈使句时,应当用动词原形.
关于ing的用法可以参见非谓语动词的用法.