★这句话里had是什么用法★

整个句子的时态由realized而决定, 整个句子在阐述一个过去发生的事情,所以have以had出现,这句话也可以这么说:
Mrs.Richards had realized that it must be the man from the Electricity Board who came to read the metre.
从口感上,用原句的比较多.

不好意思，必须要说jzpng讲错了:你把两个动作发生的时间颠倒了。

Mrs. Richards realized that it must be the man from the Electricity Board who had come to read the metre.
这是一个主从复合句。主句谓语动词realize发生在过去，而其宾语从句中的动作come to read还发生在realize之前，发生在过去的过去的动作，英语中用过去完成时来表达。
过去完成时的结构：had+v.ed(过去分词)
e.g. His father had died before he was born.
I went home early yesterday because I had been feeling terrible the whole morning.

这个句子是过去式，had就是过去的过去呀！

had to 是have to的过去时阿，
had就是have的过去时阿