1） be +现在分词,构成进行时态,例如：
They are having a meeting. 他们正在开会.
English is becoming more and more important.英语现在越来越重要.
2) be + 过去分词,构成被动语态,例如：
The window was broken by Tom.. 窗户是汤姆打碎的.
English is taught throughout the world. 世界各地都教英语.
3） be + 动词不定式,可表示下列内容：
a.表示最近、未来的计划或安排,例如：
He is to go to New York next week.. 他下周要去纽约.
We are to teach the freshpersons. 我们要教新生.
说明：这种用法也可以说成是一种将来时态表达法.
b.表示命令,例如：
You are to explain this. 对此你要做出解释.
He is to come to the office this afternoon. 要他今天下午来办公室.
c. 征求意见,例如：
How am I to answer him? 我该怎样答复他?
Who is to go there? 谁该去那儿呢?
d.表示相约、商定,例如：
We are to meet at the school gate at seven tomorrow morning. 我们明天早晨7点在校门口集合.