A person who _does__research in chemistry 中,为什么要用does

A person who _does__research in chemistry 中,为什么要用does
Finally,those who play truant are actually _taking_ up precious resources for college education,which means that others who _do_ want to receive college education are denied the chance.
一.在此句中,为什么要用do 什么时候可以省略do
二.为什么要用taking,而不用动词原形take或过去分词took
三.解释一下动名词(Ving)放句首与动词原形放句首的区别.何时可以用动名词放句首,何时用动词原形放句首?
在这个句子中：The overwhelming majority reach the same end-----parting just before graduation,forced by reality.其中又为何用parting 而后面的并列分句中为何用分词forced 是省略了Be动词么？翻译这2个长长的复合句，（Finally,those who play truant.....the chance.) 和 (The overwhelming majority ........forced by reality)