unless 是“除非”的意思,接从句
e.g My boss can't fire me unless he can prove me unsuitable for the job.
except,expect that,都是“除了”的意思
except接名词.e.g everybody is crying, except you.
except that接从句 e.g The plan is perfect, except that it exceeds our budget.
except for 是“要不是因为……”的意思额.e.g She would have travelled more except for lack of money.
besides 有两种用法.
1、“除了”.这个时候,besides一定要紧跟在句子设定的范围之后,例如在“expect”的例子中,如果用besides就要说 everybody besides you is crying.而且“except”有比较强的“唯一一个特例”的感觉.
2、besides也可以用作连词,标转折,用于句首.