这里为什么要用TO,语法上如何解释

题目

这里为什么要用TO,语法上如何解释
1 what was the purpose of the study
(1) to analyse the dynamic interaction between people and resources and the process of degradation the rehabilitaion of the watershed
(2) to assess the local socio-economic conditions
在(1)(2)一开始,句子的动词前面都加to了,例:to analyse o assess 为什么,用动名词可以吗?
请详解之

答案

下边这两句话其实是对问题what was the purpose of the study 的回答,完整的应该是：
Firstly,the purpose is to .
Secondly, the purpose is to.
在此是简略回答,也便于理解,所以就简化为：
(1) to analyse the dynamic interaction between people and resources and the process of degradation the rehabilitaion of the watershed
(2) to assess the local socio-economic conditions
希望你能明白~~~