过去进行时的句子结构为“主语 + was/were + doing”，其否定句结构为“主语 + was/were + not + doing”。以下是一些例句：
1. 肯定句：
We were having supper when the phone rang.
They were playing basketball at the park.
I was reading a book when the teacher came in.
2. 否定句：
This time yesterday, Jack was not watching TV； he was repairing his bike.
She was not studying last night.
They were not playing soccer when it started to rain.
3. 疑问句：
Were you playing basketball at four yesterday afternoon？
What were you doing when the phone rang？
希望这些例句能帮助你更好地理解和掌握过去进行时的用法。
