1．从句动作在主句动作前发生，只用 when。 ①When he had finished his homework，he took a short rest． （finished先发生）当他完成作业后，他休息了一会儿。 ②When I got to the airport，the guests had left． （got to后发生）当我赶到飞机场时，客人们已经离开了。