be动词的用法：做助动词
助动词be 有两个基本用法，一是与谓语动词一起构成各种时态，二是构成被动语态。
1、“be + 现在分词组”成各种进行式时态。例如：
Tony's maid is washing his new car.
The children are playing in the field.
Samuel was eating when I came in.
We have been living here since 1959.
This time next day they will be sitting in the cinema.
