Abraham Lincoln,who led the United States through these years,was one of the greatest presidents.

为何用who和through

先行词是林肯，以who引导一个定语从句，定语从句中缺主语，所以用who
之所以用through,是因为lead through，的意思是领导
led the United States through these years=led through the United States these years

who 引导非限制性定语从句,一般非限制性定语从句不用that 做关系代词. who 在从句中,代替先行词做主语。 through 是介词， 从一段时间开始到结束，从一个事件开始到结束的意思。