动名词和不定动词!"Playing" computer games too much is bad for your e

动名词和不定动词!
"Playing" computer games too much is bad for your eyes.
这句话为什么要用Playing而不用To play
动名词和不定动词的区别是什么?
什么时候该用哪个?

动词作主语时,就是在句首 要加ing 【除了祈使句】,
不定动词 动词不定式的句法功能非常广泛,在句中可作主语、宾语、补足语、表语、定语及状语等成分.
一、作主语
⑴动词不定式作主语时,谓语动词常常用单数.例如：
To do morning exercises is useful for our health.做早操有利于我们的健康.
⑵如果动词不定式太长,常常用 it 作形式主语,而将真正的主语——动词不定式后置.例如：
It is very important to master a foreign language.掌握一门外语是重要的.
二、作宾语
⑴能够接动词不定式作宾语的动词常用的有 ask,agree,beg,decide,determine,fail,hope,manage,offer,plan,prepare,pretend,promise,refuse,wish 及 would like/love 等,但 finish,enjoy,miss,appreciate,mind,advise,suggest 等动词后面通常只能接动名词作宾语.例如：
I hope to visit this place again.我希望能再度访问此地.
⑵动词不定式与名词构成复合宾语时,通常要用 it 作形式宾语,而将真正的宾语——动词不定式后置.例如：
I think it our duty to obey the laws.我认为遵守法律是我们的义务.
三、作补足语
某些动词在主动式中后接动词不定式作宾语补足语；在被动式中,由于原来的宾语变成了主语,故原来的宾语补足语变成了主语补足语.这类动词常用的有 consider,expect,tell,want,warn,wish等.例如：
They told him not to be late again.他们告诉他不要再迟到了.→He was told not to be late again.
四、作表语
动词不定式往往放在系动词（通常是 be）的后面作表语,而 become,sound,taste 等系动词后面一般不接不定式作表语.例如：
To see is to believe.眼见为实.
Your job today is to clean the playground.你今天的工作是打扫操场.
五、作定语
⑴动词不定式常常放在名词或不定代词后面作后置定语,其逻辑主语往往是句子的主语,故动词不定式作定语时往往用主动式(例①)；如果动词不定式的逻辑主语不是句子的主语,该动词不定式要用被动式(例②).例如：
①Do you have anything to do tonight?你今晚有什么事要做吗?
②I am going to Shanghai tomorrow.Do you have anything to be taken there?明天我要去上海.你有什么东西要捎去的吗?
⑵动词不定式作定语往往表示尚未发生的动作,如果动作已发生或正在发生,一般用现在分词作定语.例如：
The man standing there is Li Ming.站在那里的那个人是李明.
六、作状语
⑴动词不定式作状语可表示目的、原因及结果等,其逻辑主语就是句子的主语,因此动词不定式作状语往往用主动式.例如：
Come to see me again soon.尽快再来看我.
I trembled to think of it.我一想到那件事就不寒而栗.
You couldn't do that to save your life.你即使为了救自己也不能那样做.
(2)only to do sth.与 only doing sth.都可作表示结果的状语,区别是：only to do sth.表示一个与主语愿望相反的或出乎主语意料的结果,或用来暗示最初的未能实现的动作；only doing sth.表示谓语动词本身的动作造成的结果.例如：
I worked hard,only to fail at last.我努力工作,结果最后却是失败.
He died,only leaving nothing but debts.他死了,只留下一身债.