过去进行时(The past progressive tense)
构成：be的过去式+现在分词（注意现在分词的构成）
（1）过去进行时主要表示过去某个时刻或某段时间正在进行的动作。过去进行时和一般过去时经常同在一个句子里使用。过去进行时表示过去正在进行的动作或情况，一般过去时则表示比较短暂的动作或事件。正在进行中的动作或情况往往由连词when, while, as, just as等引导：
While/ When/ As I was sweeping the floor, I found your pen.
我扫地的时候，发现了你的钢笔。
while通常只引导持续时间较长的动作，并且主句和从句可以同时用过去进行时，而when和as一般不这么用：
John was watching TV while his wife was talking with a neighbour over the phone.
当他的妻子和一位邻居在电话中聊天时，约翰一直在看电视。
Some students were dancing and singing while others were eating and drinking.
有些学生在唱歌跳舞，而另一些则在吃喝。
Just as I was opening the front door, the telephone rang.
我正开前门的时候，电话铃响了。
Just as I was leaving the house, you came in.
你进来的时候我正要出门。
when也可以引导比较短暂的动作，while和as则一般不行：
We were having supper when the lights went out.
我们正在吃晚饭时灯灭了。
（2）过去进行时往往与表示一段时间的状语连用，如all morning, all night, all day, all evening, the whole week等，表示这段时间一直在干什么：
What were you doing all evening? I couldn't find you anywhere.
你整个晚上都干什么？我到处都找不着你。
I was visiting Mary.
我去看望玛丽了。
They were preparing for the party for two whole days.
整整两天他们一直在为晚会进行准备。
