过去完成时强调：“过去的过去”.表示在过去某一动作之前发生、或完成.注意上下文,肯定可以找到一个过去时态的谓语动词.那为什么不用过去式呢？过去完成时不是有个与过去时间的对比呢 ，这没有啊Dentists always ask questions when it is impossible for you to answer. My dentist had just pulled out one of my teeth and had told me to rest for a while. I tried to say something, but my mouth was full of cotton wool. He knew I collected match boxes and asked me whether my collection was growing这位是课文的前半段