The planes had been planting seed for nearly a month when it began to rain.
问题描述：
The planes had been planting seed for nearly a month when it began to rain.
had been planting 为甚么用过去完成进行时,如果用过去完成时可以吗
答
不可以,因为过去完成进行时能描述plant seed这个动作从过去一直持续到began to rain这个时间,而且在下雨的过程中也是继续的；如果用过去完成时的话,就表示下雨之前,动作就停止了.
：）