过去进行时的肯定句例句包括：

1. I was studying English when my friend called me. （当我的朋友给我打电话时，我正在学习英语。）

2. She was cooking dinner while her husband was watching TV. （她正在做晚餐，而她的丈夫则在看电视。）

3. They were playing basketball at the park this morning. （今天早上他们在公园里打篮球。）

4. The students were listening attentively to the teacher's lecture. （学生们正在认真听老师的讲座。）

5. He was singing when I arrived. （我到达时他正在唱歌。）

6. We were having class at this time yesterday. （昨天这个时候我们正在上课。）

7. They were playing chess at 8 o'clock last night. （昨晚8点他们正在下棋。）

8. I was wondering about it. （我对那事感到疑惑。）

9. All the pupils were writing. （所有的学生都在写作。）

10. We were having supper when the phone rang. （我们正在吃晚饭时，电话响了。）

这些例句展示了过去进行时在不同情境下的应用，包括过去某个时刻正在进行的动作、与时间状语连用的例句等。希望这些例句能帮助你更好地理解和运用过去进行时。