过去完成进行时表示动作在过去某一时间之前开始，一直延续到这一过去时间。和过去完成时一样，过去完成进行时也必须以一过去时间为前提租信。I had been looking for it for days before I found it.这东凳行西我找了好枣型哗多天才找着的。They had only been waiting for the bus a few moments when it came.他们只等了一会儿车就来了。