At one time,there were long queues of people waiting outside the CAAC offices.中的waiting怎么用?

问题描述:

At one time,there were long queues of people waiting outside the CAAC offices.中的waiting怎么用?
前面有个there were,是there be 句型么?后面的waiting是不是和前面的were构成过去进行时,感觉were被两个句型所用.是不是短一个were.自己瞎想.waiting在句中是个什么成分.
为什么要用waiting而不用wait呢?

这里的waiting ---是个伴随状语,就上那群人现在的伴随状态,跟前面的there be 没关系