its / it's
its: possessive determiner similar to 'my' or 'your'
its：物主代词，和“我的”或者“你的”是一类的。
Examples:
例句：
Its color is red.
The dog didn't eat all of its food.
它的颜色是红色。
这只狗没有把它所有的事物吃掉。
it's: Short form of 'it is' or 'it has'
it's：it's是it is或者it has的缩写。
Examples:
例句：
(it is) It's difficult to understand him.
(it has) It's been a long time since I had a beer.
（it is）要理解他太困难了。
（it has）我很久没喝过啤酒了。
