But Expressions

Nothing but / All but

NOTHING BUT	ALL BUT
Nothing but is an expression meaning "only".  It emphasizes the noun after it.	All but expresses an exception or only this is left. (one, two or little)
Jack eats nothing but hamburgers. only hamburgers, lots of them	He ate all but one hamburger.  except
There was nothing but sugar in the "energy bar".  only sugar, lots of it	They sold all but two energy bars.  except
He was nothing but trouble.  only trouble, lots of it

If not for / But for

IF NOT FOR	BUT FOR
If not for expresses the idea of "if something had not existed/happened"	But for expresses the idea of "if something had not existed/happened".
If not for traffic, I would have been home an hour ago.	But for traffic, I would have been home an hour ago.
If not for your help, I would never have finished on time.	But for your help, I would never have finished on time.

Who… but

BUT FOR	WHO…BUT
But for expresses the idea of "if something had not existed/happened".	Who / What should...but  is used to express surprise at a chance meeting or finding.
I was walking down the street, and unexpectedly I saw Jack.	I was walking down the street, and who should I see but Jack.  a chance meeting
I looked down the street, and luckily I found a parking space!	I looked down the street, and what should I find but a parking space!   a lucky find

Cannot… but

CANNOT HELP + VERB-ING	CANNOT BUT + VERB
A gerund is used after can't help.   Can't help means cannot avoid or cannot stop from doing.	Cannot but expresses the idea of "we can do nothing except ..." (formal).  The expression cannot help but is a combination of can't help and cannot but. It is informal.
We can't help admiring his courage.	We cannot but admire his courage.  very formal We cannot help but admire his courage.  very informal (not for academic or business use) We admire his courage very much.
Jack can't help drinking.	We could not but see that he was drunk. We couldn't help but see he was drunk. We saw clearly that he was drunk.