skim就是 read sth quickly in order to find a particular point or the main point即快速阅读以找到文章的中心思想/主要意思.
If you skim through the play too quickly,you'll forget the plot.
如果你读剧本读得太快,就会忘记剧中主要情节.
scan就是 look at every part of sth carefully,especially because you are looking for a particular thing or person即快速阅读以找到某一具体的信息
His mother scanned his face to see if he was telling the truth.
他母亲察看他的面色看他是不是在讲真话.
scan的意思：细看; 扫描; 粗略地看; 审视; 浏览; 符合格律,韵律合拍; 扫描,扫掠; 标出诗的格律