关于新概念第二册第42课的时态As we had had a long walk through o

网友回答

两个或两个以上相继发生的动作,用and, but, so, as soon as 按动作发生的先后顺序连接,此时要用一般过去式,不用过去完成时,例如原文中：It rose out of the basket and began to follow the movements of the pipe. （rose out of 和 began to follow）;
此外,一般过去时的表示两个相继发生的动作时,有时两个动作之间不是孤立存在,而是存在一定的逻辑关系,常见句so, as soon as 连接的句子中,如：
we noticed a snake-charmer with two large baskets at the other side of the square, so we went to have a look at him. 由于注意到了玩儿蛇的人,所以过去看看 （因果关系）
As soon as he saw us, he picked up a long pipe which was covered with coins and opened one of the baskets. 他一看见我们,于是就拿起一个长管子,并且打开了一个篮子的盖子（saw和后面的并列结构中的picked up 与 opened 存在因果关系）；
When he began to play a tune, we had our first glimpse of the snake.
当玩儿蛇人开始奏曲时,我们才第一次看见那条蛇,play a tune 致使 have a glimpse of（ 看见）the snake, 奏曲与看见呈因果关系；
We were very much surprised when the snake charmer suddenly began to play jazz tunes and modern pop songs 玩儿蛇人开始演奏爵士和现在流行乐时,我们特别吃惊,演奏Jazz tuens and modern pop songs 导致 surprised, 还是因果关系.
The snake, however, continued to 'dance' slowly.
however作用等同于but,continued to 强调动作持续,蛇之前是began to follow the movement,
因此时态必然一致,使用一般过去时；
最后讲下第一句为何使用过去完成时,As we had had a long walk through one of the markets of Old Delhi, we stopped at a square to have a rest. 这句话意思为,当我们穿过Old Delhi 市场
走了很长一段路,我们在一个广场停了下来休息.
这里是过去完成时态的持续性用法,即站在过去一个时间点（停下来休息）,去回顾该时间点之前发生的一个持续性的动作（had had a long walk）,该持续动作一直持续到之前的时间点（stopped）结束(完成);
举一反三： There had been sixty universities in our city up till 2003.
He served in the army for five years; then he retired and married.
I had waited for her for tow hours in such a severe winter when she eventually turned up.