some of them 为什么要用them?

some of them 为什么要用them? some of them怎么用 Both of them study very hard.为什么要用them? many of them are young and some are old 为什么要用some 请问 some of them are students 这句话当中为什么用them而不用they? Some of them are some of them are red.some of them are yellow. some of them怎么读 Here are some of them. some of they还是them? both them还是both of them?为什么? 英语改错题：maybe some of them do feel thankfully,为什么要将thankfully改