过去进行时句型：主语+be（was，were）+现在分词～.

He was walking in the park at that time yesterday

昨天那个时候，他正在公园散步。

I was watching TV then.

那时，我正在看电视。

When I came home， they were cooking in the kitchen.

当我进家时，他们正在厨房里做饭。