1. be +现在分词，构成进行时态。如：
They are having a meeting. 他们正在开会。
We were planting trees this time yesterday. 我们昨天这个时候在植树。
2. be + 过去分词，构成被动语态。如：
The door was broken by Tom. 门是汤姆打破的。
English is taught in our school. 我校教英语。
3. be + 动词不定式，可表示：
A. 打算、决定、计划、安排等。如：
He is to visit Japan next week. 他决定下周访问日本。
I was to have told you yesterday, but you were not in. 我本打算昨天告诉你的，但你不在家。
B. 指示、命令、禁止等，即“必须，一定”。如：
You are to hand in the exercises tomorrow. 你们明天必须交练习。
You're not to (=You mustn't) smoke here. 你不可以在此处抽烟。
C. 责任、义务、需要，即“该不该”。如：
You are to (=should) report to the police. 你应该报警的。
Am I to go on with the work? 这工作我该不该继续干下去?
D. 可能性，与can, may相当，不定式多用被动式。如：
The book was not to (=could not) be found. 那本书可能找不到了。
E. 假设，用于虚拟条件句中。如：
If it were to rain, we would get wet. 要是下雨，我们就会淋湿。
F. 表示注定要发生或后来必然发生。如：
This experience was to change his life. 这次经历注定要改变他的生活。
注：不定式作表语与上述用法是不同的。如：
My idea is to go there today. 我的意思是今天就去那里。
