please be very quiet

祈使句中,please 后要接动词原形.

be very quiet中, be 是这个词组的动词. 所以用 be

如果在陈述句中,要根据主语变的。如

He is very quiet.
I am very quiet.
They are very quiet.

quite 是形容词，前面需要BE动词

否则一句话中没有动词了啊~
祈使句+动词原形~所以就是BE了

quiet是形容词啊，没有be动词就无法构成祈使句啦。