are:主语为 you,we,they以及其他所有复数名词
You are nice.
They are nice.
Apples are delicious.
is:主语为第三人称单数
It is nice.
The tiger is dangerous.
He is smart.
be:Don't be shy
Be brabe.
been:be 的过去分词,用于过去完成时等
I have been to that city.
I have been reading for one hour.
(这个理论挺难概括的)