Let them join us中,为什么them要用宾格不能用“they”?

Let them join us中,为什么them要用宾格不能用“they”? I hope __(them to,they can )join us Let's go and join them,_____ let us wish them success!中的success为什么不用successsful? .Let them help us with English为什么用help They need us( ) (help)them. 英语翻译They are kick a ball Let's go and join them. fun they are having!Let's join them.A.What B.How C What a D. would join them in the dress-up party __.A.if they invite us let us help ___.they are in trouble .A they B them C their let----help-----.a.they ,you b.us,they c.her,them d.we,you -Shall we __ the dancing?-Good idea.Let's __ them.为什么填join i