1, 很乖就可以说, very good, 或者well-behaved, 行为举止好, 不出错, 可以理解为乖
2, that child is very naughty sometimes, but he is very good occasionally.
3, 听话,也有用listen,不是不可以,还可以用obey,或者follow,都可以的,看你喜欢哪个
4, today, it is his 25 years birtday. Many people come to participate in/take part in his birthday party.
5, how old are you on tomorrow?
6, it is my 20 years old birthday. i want to invite you to attend my birthday party.