You're supposed to pay the bill by Friday

最好填 had better +V原 表示你最好怎样 supposed to+ V原 表示应该或被希望...
意思很相近
如果说话人的口气很强硬，还可以填 had to

You're supposed to pay the bill by Friday
改同义句
You （had ） （better ） pay the bill by Friday
我是英语教师，有不明的再问我吧。

were to