怎样判断to后面到底是动词原形还是动名词？

是不定式结构to do结构,用动原；如果to单独作为一个介词，要用动名词。

只要to是介词,后面就要接动名词.如:

be used to doing,习惯于；object to,反对；devote oneself to,致力于；stick to,坚持；pay attention to,注意；respond to,回答；look forward to,期盼；see to,照料；contribute to,帮助；Submit to,服从；adapt to,适应；apply to,运用；accede to,同意,就任；prefer to,倾向于；adjust to,使自己适应于；owe to,归咎；react to,作出反应；access to 接近,进入(某地的)方法，通路；according to 按照，依照视....而定；be addictedto 沉溺于.........上瘾等。

不定式结构to do结构,用动原等，如：

afford to do sth.负担得起做某事；agree todo sth.同意做某事；arrange to do sth.安排做某事；pretend to do sth.假装做某事；promise to do sth.答应做某事；refuse to do sth.拒绝做某事；want to do sth.想要做某事；wishto do sth.希望做某事；aim to do sth.打算做某事、fail to do sth.未能做某事；long to dosth.渴望做某事；happen to do sth.碰巧做某事；hesitate to do sth.犹豫做某事；struggleto do sth.努力做某事；advise sb to do sth建议某人做某事；allow sb to do sth.允许某人做某事ask sb to do sth.请(叫) 某人做某事；beg sb to do sth.请求某人做某事；cause sb to do sth.导致某人做某事；command sb to do sth.命令某人做某事；encourage sb to do sth.鼓励某人做某事等。