他在监狱呆了二十年,出狱后,他对周围的变化感到很惊奇.
when he was set free after twenty years in prison 是表示时间的从句；
was set free：被释放 in prison在监狱
he was amazed at the changes he found.是句子的主体部分.
was amazed at：对···感到惊奇.he found修饰the changes
就是这样!