关于新概念第二册第42课的时态

关于新概念第二册第42课的时态
As we had had a long walk through one of the markets of Old Delhi,we stopped at a square to have a rest.After a time,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.As soon as he saw us,he picked up a long pipe which was covered with coins and opened one of the baskets.When he began to play a tune,we had our first glimpse of the snake.It rose out of the basket and began to follow the movements of the pipe.We were very much surprised when the snake charmer suddenly began to play jazz tunes and modern pop songs.The snake,however,continued to 'dance' slowly.It obviously could not tell the difference between Indian music and jazz!
在第一句之外,文中很多句子中描述的动作是有前后的,为什么没有再用到过去完成时态?比如第二句“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.”,noticed 应该是在"went to have a look at hime" 之前发生的,即是在“过去的过去”发生的,为什么不用过去完成时?希望能逐句把时态讲解一下,