过去进行时
(一)定义
过去进行时表示在过去某一时刻或某一段时间内正在发生或进行的动作或状态。如：
A fire was burning in the grate. (火在壁炉中燃烧着。)
Fish were surfacing to catch insects. (鱼浮到水面上捕捉昆虫。)
(二)基本结构
过去进行时的句子结构一般为：
主语+was/were+现在分词(doing)+其他成分
如：
The wind was whispering in the trees. (一阵风穿过树林沙沙作响。)
The boys were itching for the lesson to end. (那些男孩子们渴望下课。)
(三)主要用法
1、表示在过去某个时刻发生的事情。该时刻可以用介词短语、副词或从句来表示。如：
The children were playing in the sand pit at four o'clock yesterday afternoon. (昨天下午四点孩子们正在沙坑中玩耍。)
I was reading a magazine when the phone rang. (电话铃响时我正在看一本杂志。)2、表示在过去某个时间段发生的事情。此时往往会有较为明确的过去时间段的时间状语。如：
She was reading in the library the whole afternoon last Saturday. (上周六她整个下午都在图书馆看书。)
They were playing football on the playground this morning. (今天上午他们一直在操场上踢足球。)3、 在复合句中，如果主要动作和背景动作都是延续的或同时发生的，那么主从句的动词都可用过去进行时。但是，如果背景动作是短暂性动作，而主要动作是延续性动作的话，则背景动作用一般过去时，而主要动作用过去进行时。前者复合句中的从句较为典型的连接词是when，后者较为典型的连接词是while。如：
I was paging through a magazine when he came in. (他进来时我在翻阅一本杂志。)
Yesterday afternoon, while Jack was working in the garden, she was making a cake in the ktichen. (昨天下午，当杰克在花园里干活时，Mary在厨房做蛋糕。)
(四)特殊用法
1、不能用于进行时的动词
agree, be, believe, belong, care, forget, hate, have(拥有)，hear, know, like, love, mean, mind, notice, own, remember, seem, suppose, understand, want, wish等。如：
错误：I was hearing that they would get married soon. (听说他们要结婚了。)
正确：I heard that they would get married soon.2、表过去即将发生的事情
此时的谓语动词一般都是come、go、leave、start、arrive等表示位置转移的动词。如：
She told the manager she was leaving next month. (她告诉经理，她下个月就要走了。)3、表过去习惯或爱好
与always、constantly等频度副词连用，表示感*彩，往往暗示某种习惯或爱好。如：
She was always talking a lot before she got married. (她在婚前总喜欢说个不停。)4、when、while和as的区别
请参考这篇文章：英语时间状语从句的用法大全
(五)标志性词语
过去进行时常见的标志性时间状语主要有：this morning、the whole morning、all day、yesterday、from nine to ten last evening、when、while、at that time等。如：
She was drinking all day by the dower. (她用从亡夫处得来的产业整天喝酒。)
We were standing on the right at that time. (我们当时站在右边。)
(六)句型转换
因为“was/were doing”中的was/were是助动词，所以在进行句型转换中要注意不需要再另外借助助动词do/did/does，而是直接借用was/were。如：
肯定句：I was trying to work out a solution to the problem at this time yesterday.
否定句：I wasn't trying to work out a solution to the problem at this time yesterday.
一般疑问句：Were you trying to work out a solution to the problem at this time yesterday?
特殊疑问句：What were you trying to work out at this time yesterday. (划线部分为a solution to the problem)