语法点：过去进行时 (Past Continuous)
过去进行时表示过去某一时刻或某一段时间里正在进行的动作，像给故事拉开一个“背景幕布”：侦探们整个上午都在等（were waiting），飞机到达（arrived）这个短动作就发生在这个背景之中。它最常和一般过去时搭配：长的、持续的动作用过去进行时，短的、打断它的动作用一般过去时，用 when 或 while 连接。while 后面通常跟过去进行时，when 后面通常跟一般过去时。
结构
| 形式 | 结构 | 例句 |
|---|---|---|
| 肯定 | 主语 + was/were + 动词-ing | They were waiting at the airport. |
| 否定 | 主语 + was/were + not + 动词-ing | They were not waiting inside. |
| 一般疑问 | Was/Were + 主语 + 动词-ing? | Were they expecting a parcel? |
| when 句型 | When + 一般过去时, 过去进行时 | When the plane arrived, they were waiting. |
| while 句型 | While + 过去进行时, 一般过去时 | While they were keeping guard, others opened the parcel. |
I/he/she/it 用 was，you/we/they 用 were；动词-ing 注意拼写：wait → waiting，expect → expecting，keep → keeping。
课文中的例句
- [句 1] The plane was late and detectives were waiting at the airport all morning. — were waiting + all morning：整个上午持续进行的动作
- [句 2] They were expecting a valuable parcel of diamonds from South Africa. — were expecting 交代故事背景：他们当时正在等什么
- [句 4] When the plane arrived, some of the detectives were waiting inside the main building while others were waiting on the airfield. — arrived 是短动作，were waiting 是当时正在进行的背景
- [句 6] While two detectives were keeping guard at the door, two others opened the parcel. — while + 过去进行时，主句 opened 用一般过去时
- [句 5] Two men took the parcel off the plane and carried it into the Customs House. — 对比：一个接一个完成的动作，用一般过去时
补充例句
- I was waiting for the plane when a detective walked up to me. 一个侦探朝我走过来时，我正在等飞机。
- While the two men were carrying the parcel, the thieves were watching them. 那两个人搬包裹的时候，小偷正盯着他们。
- What were you doing at nine o'clock yesterday morning? 昨天早上九点你在做什么？
易错点
- ❌ Detectives was waiting at the airport. → ✅ Detectives were waiting at the airport. 复数主语要用 were。
- ❌ They were expect a parcel. → ✅ They were expecting a parcel. was/were 后面必须接动词-ing。
- ❌ When the plane was arriving, they were waiting. → ✅ When the plane arrived, they were waiting. arrive 是瞬间动作，打断背景的动作用一般过去时。
- ❌ While two detectives kept guard at the door, two others were opening the parcel. → ✅ While two detectives were keeping guard at the door, two others opened the parcel. while 引导的是持续的背景动作，用过去进行时；主句的短动作用一般过去时。