he works as hard as he can to afford his children,这样可以的.按理来说是要用动词原形的.但是像一楼的朋友说的这种情况,把as hard as he can看做一个修饰成分,修饰work来突出他工作的辛苦程度,这样把这个成分去掉再看,是he works to afford his children.这样用不用动词原形就与can没什么关系.