They were playing football when it started to rain.

开始下雨的时候他们正在踢足球。

同一单元的其他句子

• She has been studying English for five years.她已经学了五年英语了。
• They will have finished the project by next Monday.到下周一他们将已经完成这个项目。
• I was reading a book when the phone rang.电话响的时候我正在看书。
• He had left before I arrived at the station.我到车站之前他已经离开了。
• We are going to visit the museum tomorrow.我们打算明天去参观博物馆。