什么时候用be动词的原形

Be
【化】元素铍( beryllium )的符号

be
[美国] [bi] [英国] [biː] 动词听任
If she's sleeping, let her be. 如果她在睡觉,就别惊动她。

存在
Whatever is has every reason for being. 存在的东西总有其存在的理由。

在[Q]
Tony is in the office. 托尼在办公室里。

逗留,(继续)呆[L]
He will be here all next week. 下周他将一直呆在这儿。

(与 there 连用)有[L]
There are a lot of tourists there. 那里有许多观光客。

(常用祈使语气或不定式)要,得;成为;变成[L]
Be quiet, please. 请安静。

(表示时间、度、量、价值等)是;值;等于[L]
The watch is 100 dollars. 这表值一百元。

是[L]
At that time he was the chair of the department. 当时他是系主任。
I am a teacher and they are my students. 我是老师,他们是我的学生。
She is very young. 她非常年轻。

(与动词的现在分词连用,构成各种进行时态)正在...,在...
I won't be free this afternoon. I'll be seeing a friend off. 我今天下午没空,我要给朋友送行。
What were you doing from 9 to 11 this morning? 今天上午九点到十一点你在做什么?
He is working. 他在工作。

(与动词不定式连用,表示职责、义务、意图、约定、可能性等)应该做...,