Two men who recently spent five days on a coral island wished they had stayed there longer.

请问这个句子中,为什么用完成时态的 had stayed 呢, 这样体现不出"能"的意思吧?  

"希望能够呆更久",为什么不使用could stayed 呢?

     【答】在 wish 后面的宾语从句中，过去的愿望要用过去完成时来表示。如：

      I wish you hadn't said that. (=It would be nice if you hadn't said that. )你要是没有说过那样的话该多好。（摘自《牛津英语用法指南》第四版632.4）