不定式
不定式作定语时，与被定词之间存在某种逻辑关系，多数情况下可以把该不定式短语扩展成为一个定语从句------即不定式作定语可以看作一个定语从句的缩略形式：
1.主谓关系（主动被动）：不定式修饰的名词是不定式表示动作的逻辑主语：This is a special purpose computer to solve only one kind of problem(约等于which can solve only one kind of problem), computer/solve的主动的主谓关系；
The television camera scans the subject to be transmitted(约等于which will be transmitted),the subject/to be transmitted之间存在被动的主谓关系。
2.动宾关系（不定式只能为主动形式；前面被修饰词为不定式所表示动作的逻辑宾语，动作发出者为主语或者任何人）： “weight” is a difficult thing to define(约等于that one can define),thing 是define 的对象；
3.不定式短语末尾的介词与其介词宾语的关系：这类不定式可以扩展为“介词+which”开头的定语从句：There are many types of tools to choose from(约等于from which one can choose);在正式科技文中，这类不定式经常以 “介词+which”开头。
4.纯修饰关系：不定式主动修饰抽象名词：ability/aim/capability/power/puepose/objective/use/tendency/chance/opportunity/time/attempt/way) ；There are several ways to take an average.取平均值的方法有几种。
5.若不存在主谓或动宾关系，应使用介词+which+to(do):We have defined mass units with which to measure m.
