i knew that my turn had come.这是新概念二上面的一句话,为什么用had+动词原型这个用法?

问题描述:

i knew that my turn had come.这是新概念二上面的一句话,为什么用had+动词原型这个用法?
就是不明白had为什么加动词原型啊

come的过去分词还是come.
come came come