the experts were now fully convinced that the animal was a puma,but where had it come from?As no pumas had been reported
问题描述：
the experts were now fully convinced that the animal was a puma,but where had it come from?As no pumas had been reported missing form any zoo in the country
but where had it come from?这里的had为什么要用过去完成时呢 ,而且come from这里的come为什么不用came?
答
发生在过去的过去 用过去完成时 既然是过去完成时 had+过去分词 自然是had come from came 是come的过去式哦 肯定是不能用在这里的 原形come---过去式came---过去分词come