Why not ___(LOOK)at the sign?You shoulden't____(smoke)here.

looking why +doing
have smoked should +虚拟语气

look smoke

look smoke
1、why not后面加原型
2、should是情态动词，后面加原型

look
have smoked (should +虚拟语气),意为你本不应该。现在烟已经抽了。