我在新概念3第五课看到这么两句句子The article began: 'Hundreds of steps lead to the high wall which surrounds the president's palace'.
However, he had at last been allowed to send a fax in which he informed the editor that the he had been arrested while counting the 1,084 steps leading to the fifteen-foot wall which surrounded the president's palace.
为什么第一句用surrounds 而第二句用surrounded啊？