★过去进行时主要用于如下几类情况：

1.过去某一时刻正在发生的动作

♟when he called me, I was having dinner.

♟my father was watching TV while my mother was cooking dinner.

2.过去某个时间段内正在发生的动作

♟Lisa was working on her composition yesterday afternoon/the whole morning.

♟we were studying the project last semester.

3.为过去的动作提供背景

♟it was a cold dark night and it was raining hard.

♟when I was taking a walk yesterday afternoon, I met a friend of my mother's in the park.

4.与副词连用，表示过去的不良习惯

♟the twins were often quarreling when they were young.

♟In Qing Dynasty, the government was always making concessions to western powers.

长风破浪会有时，直挂云帆济沧海。