the population of china is much larger than of japan

the population of china is much larger than of japan
life is much easier than used to be
为什么第二句中用it?可以替换成that么?
life is much easier than (it) was in the 20 centry
life is much easier than (that) in the 20 centry
为什么多加一个WAS用的代词就不一样了？？
it和that有什么准确明确的区分方法么？
that是指Japan的population 而第二句，it就指从前的life
这两个不都不是指同一样东西吗？ 还是不懂啊？