it isn't 强调在it上，强调是“它”不是，不是别的东西不是，it's not相反，强调是“不是”。

its not和it isnt用法是一样的，无太大差别。

例句：

Its not nearly as hot as last year.

天气绝没有去年那么热。

It isnt always easy to separate the wheat from the chaff.

分清好坏并不总是容易的。