①Before the last bus came, I had been waiting for half an hour.
②Until then she had been living with her daughter.
③I didn't know that you had been looking for that book.
④It had been raining and the street was still wet.
⑤His arms were sunburnt because he had been sitting in the sun all day.
⑥He felt very tired because he had been working in the field for at least five hours.
[我的发现]
(1)例句①②③④⑤⑥中均用了过去完成进行时，其结构为had_been＋现在分词。
(2)过去完成进行时表示一个动作从过去某时开始，一直延续到另一个过去时刻，而那个时刻，该动作可能刚刚终止，也有可能还在继续。
