再补充一点：

过去进行时有很多用法.

我把第四种用法 copy 在这，可能对我们的讨论有帮助。 

USE 4 Atmosphere

In English, we often use a series of parallel actions to describe the atmosphere at a particular time in the past.

Example:

• When I walked into the office, several people were busily typing, some were talking on the phones, the boss was yelling directions, and customers were waiting to be helped. One customer was yelling at a secretary and waving his hands. Others were complaining to each other about the bad service.

Here is the link for other uses: http://www.englishpage.com/verbpage/pastcontinuous.html