Some of the pyramids still look much as they must have done

这是一个多重复合句.其结构如下：
 
　　主句：Some of the pyramids still look much 一些金字塔看起来还是老样子
　　定语从句：as they must have done when they were built thousands 这一点必定在数千年前建造时就是如此,
　　·【关系代词兼宾语】as （指代前面的整个句子,等于说“这一点”或“正如前面所过的这一点”）
　　·【主语】they
　　·【复合谓语动词】must have done （虚拟语气,表示对过去事实的推测.动词done 代替与前面重复的谓语动词 looked）
　　·【时间状语】when theywere built thousands
 
基于上述分析,可以把它改写为如下的句子：
 
　　Some of the pyramids still look much; they must have looked what they are (on this point) when theywere built thousands.
 
注：

第一个look 是不及物动词,表示“显示着外表”,第二个 looked 是及物动词,表示“看起来与…一致”

副词 much 表示“差不多”或“在很大程度上”