5 You can send a letter if you know the address.
6 You can't go in unless you're a member.
You can go in if you're a member.（均可）
7 Don't write to them again unless they reply.
8 They will hold a sports meeting unless it rains tomorrow.
兄弟好好学啊 ,思考比较一下怎么变得,if变成unless得后面的否定变肯定.即为：
.如果.不怎么样.===> .除非.怎么样