一、基本区别
介词besides, but, except都可表示“除外”，其区别是：
besides 表示一种累加关系，意指“除了什么之外，还有……”；
而 except 或 but 则表示一种排除关系，意指“除了什么之外，不再有……”。如：
Besides his wife，his daughter also went to see him.
除他妻子外，他女儿也去看过他(即妻子女儿都去看过他)。
Nobody went to see him except [but] his wife.
除他妻子外，没有一个人看过他(即只有他妻子去看过他)。
注：在否定句中，besides 也表示“除……之外不再有…… ”，与but, except 同义。如：
No one passed the exam besides [except] Jim. 除吉姆外，没一个人通过考试。
上述两种含义均可用apart from：
What other sports do you like apart from football?
除足球外你还喜欢哪些运动？
I like all sports apart from football.
除足球外我喜欢所有的运动。
二、but 与 except的区别：
1) 两者都可表示“除……外不再有……”，但含义上略有差别：but侧重指意义的几乎完整性，而 except 则侧重指后面除去的部分。比较：
All are here but one. 除一个人都到了。
All are here except one. 还有一个人没到。
2) 在现代英语中， but 的介词用法十分有限，一般说来，它只能用在下列词语之后：
① no, no one, nobody, nothing, nowhere 等
② any, anyone, anybody, anything, anywhere 等
③ every, everyone, everybody, everything, everywhere 等
④ all, none 等
⑤ who, what, where 等
Everyone knows it but you. 除你之外大家都知道。
I haven't told anybody but you. 除你之外，我没告诉任何人。
No one but he [him] showed much interest in it. 除他之外没有一个人对此有很大兴趣。
一般说来，若没有出现上述词汇，就不宜使用介词 but， 否则可能造成错句。但是 except 却没有以上限制。比较：
正：The window is never opened except in summer. 除夏天外，这扇窗户从不打开。
误：The window is never opened but in summer.
说明：except可以引导介词短语或从句。but却不可以。
3) but 一定不能用于句首，except 通常不用于句首：
除我之外每个人都累了。
正：Everyone is tired but (except) me.
正：Everyone but (except) me is tired.
误：But (Except) me, everyone is tired.
注：except for 可用于句首，表示 except 的意思：
正：Except for me, everyone is tired.
三、关于 except 与 except for的区别
except 主要用来谈论同类的东西，而 except for 则主要用来谈论不同类的东西，有时含有惋惜之意。except for连接的部分所表示的与前面部分并非同一类事物，而它们之间又是所属关系，句子后半部分对半部分所说的基本情况在细节上加以修正，总是对前面句子进行修正，如：
All compositions are well written except yours. 除了你的作文外，其他的作文都写得很好。
Your composition is good except for some spelling mistakes.
除了有几个拼写错误外，你的作文整体上写得很好。
The bus was empty except for one young lady.
公共汽车上很空，只有一位年轻的女士。
His composition is good except for a few spelling mistakes. 他的作文写得不错，只是有几处拼写错误。
注：若用于句首，则 except for 与 except 同义(但 except 通常不用于句首)。
Except for her illnesses, she had had a particularly happy childhood.
除了体弱多病外，她的童年特别幸福。
Except for the remarkably tidy kitchen, the place was a mess.
除了厨房特别干净外，这地方一片狼藉。
I had nothing on except for my socks.
我除了短袜什么都没穿。
I sold everything I owned except for my car and my books.
我把我所有的东西都卖掉了，只剩下汽车和书。
The room was silent except for John's crunching.
屋子里一片寂静，只有约翰发出的咀嚼声。
The bus was empty except for one young lady.
公共汽车上很空，只有一位年轻的女士。
Apart from [ Except for] a few grammatical mistakes, this composition of yours is well written.
除了几处语法错误之外，你这篇作文写得不错。
She was unlike him in every way except for her coal black eyes.
她除了那双乌黑的眼睛外，跟他没有一点儿相像。
except后接名词，代词，动词不定式，介词短语和从句。
I didn't tell him anything except that I needed the money.
我什么都没告诉他，只是说我需要钱。
Our dresses were the same except mine was red.
我们的连衣裙是一样的，只是我的那件是红色。
We don't do much in the evenings except watch television.
我们在晚上除了看电视不怎么干别的事。
They live in a remote area, inaccessible except by car.
他们住在一处偏远地区，只能开车去。
四、except for 与 but for的区别
except for 主要表示排除，but for 主要表示一种虚拟条件(与虚拟语气连用)。如：
Except for me, everyone passed the exam. 除了我之外，大家都通过了考试。
But for my help, she would not have passed the exam. 若不是我的帮助，她就不会通过考试。
But for the atmosphere plants would die. 如果没有大气，植物就会死亡。
But for you, they might have given us the slip.
要不是你，我们可能已经被他们甩掉了。
I'd have been lost but for you.
要不是你，我就走迷路了。
But for your help, I might have failed.
假如没有你帮忙，我兴许会失败。
But for your magic hand, I'd have long kicked the bucket.
要不是你妙手回春，我早就见阎王了。
but for也可作解为：对……而言
But for me television is an escape.
但是对我而言，看电视是一种逃避的方法。