We were about to leave when it began to rain.
Be about to表示将要正要……,
翻译为：当我们正要离开的时候天开始下雨了.
was / were about to + 动词原形.如：
We were about to leave there when it began to rain heavily and suddenly.
就在我们要离开时,天突然下起了大雨.
He was about to have lunch when the bell rang.
就在他要吃中饭的时候,门铃响起来了.