老师好，
One report concludes that many schools do not have, nor are they likely to have, enough computers to use them effectively.
请问为何这里use后面要加them呢？这个结构与I have a lot of work to do. 不同在哪儿呢？前者是不定式作状语？后者是不定式作定语？
句子来源：GMAT PREP
The comet moves quickly enough for scientists to observe.
请问这个句子正不正确？需要在observe后面加上it吗？这里to observe是什么成分呢？
谢谢老师的解答！
一个不定式的动词或介词的宾语缺失（缺失的宾语在不定式前面出现——句子的主语、宾语或表语），这种不定式就叫做悬空不定式（hollow infinitive) 或反射不定式。
悬空不定式需要或可以用于6种状况。
其中之一就是不定式修饰程度副词too, enough, sufficiently时。但应该注意的是，此时悬空不定式的用法是选择性的，而不是需要的。即便用普通不定式也是可以的。比如：
The tea is too hot to drink .
以下为夸克《大全》15.73 的例句：
It moves too quickly for mostpeople to see .
The television in the apartment above was loud enough for us to hear .
My ambitions are modest enough for me to fufil .
He was old enough for us to talk to seriously.
网友的以下二个句子中都有enough，不定式是修饰enough的。
One report concludes that many schools do not have, nor are they likely to have, enough computers to use them effectively. 这个句子简化如下：
Many schools do not have enough computers to use effectively.
The comet moves quickly enough for scientists to observe .
括号表示不定式的宾语可有可无。有宾语，则为普通不定式。没宾语就是悬空不定式。
第一个句子假如没enough，谓语动词为have，则不定式与have的宾语为动宾关系时，则不定式就需要为悬空不定式，即不定式的宾语不可以出现：
The school have no computers to use. （to use 的宾语不可以出现）
不定式修饰表示难易程度的表语形容词时，不定式需要是悬空不定式：
English is easy to learn. 不可以说：English is easy to learn it.
但假如easy 被enough修饰，不定式就不再是修饰easy，而是修饰enough。此时不定式可以是悬空不定式，也可以是普通不定式。即便不定式与主语为动宾关系，不定式的宾语可以出现，也可以不出现：
English is easy enough to learn .
又如：English is difficult for me to learn. . (此时不定式修饰的是too, it可有可无）
