it have been necessary 和 it is necessary 的区别?

题目

it have been necessary 和 it is necessary 的区别?
那你帮我看下这句话好吗？这句话有点长，这里的necessary,是不是虚拟语气，如果是虚拟语气，为何前面没有你所说的would?只是it had been necessary.而不是it would have been necessary?
But rich planterswere few in the young county of Clayton,and,inorder to muster a full-strength troop,it had been necessary to raise more recruitsamong the sons of small farmers,hunters in the backwoods,swamp trappers,Crackersand,in a very few cases,even poor whites,if they were above the average oftheir class.

答案

it have been necessary 和 it is necessary 的区别
是没有什么区别的.
若有的话也是 时态上的区别
文中用it had been necessary 是过去完成时,因为 这里 if they were 是过去时.所以前面用had been 表在这个 were 之前.