什么时候是look up them ,什么时候look them up?

什么时候是look up them ,什么时候look them up? 像look=（代词）up 一类什么时候代词放中间?look them up可以,那look them for为什么不行? look them 中间是? 是look after them,还是look them after? look it up与look them up有什么区别,怎么用 翻译：pull your covers up and make them look neat 是cheer them up还是cheer up them look them look them look them look at the paper on the floor,please pick it up 还是 pick them up? If you don't know the words,you can _____in a dictionary.A.look it up B.look up them C.look them up D.look up it选择. look up look up look up look up don't be afraid of new words .you can __ in the dictionary.A.look at them B.look for them C.look them up D.look them over look up和look