but 
(bŭt; bət when unstressed)
(bŭt; bət when unstressed)conj.
1. On the contrary: the plan caused not prosperity but ruin.
2. Contrary to expectation; yet: She organized her work but accomplished very little. He is tired but happy.
3. Usage Problem Used to indicate an exception: No one but she saw the prowler.
4. With the exception that; except that. Often used with that: would have joined the band but he couldn't spare the time; would have resisted but that they lacked courage.
5. Informal Without the result that: It never rains but it pours.
6. Informal That. Often used after a negative: There is no doubt but right will prevail.
7. That ... not. Used after a negative or question: There never is a tax law presented but someone will oppose it.
8. Informal Than: They had no sooner arrived but they turned around and left.
prep.
Usage Problem Except.
adv.
1. Merely; just; only: hopes that lasted but a moment.
2. Used as an intensive: Get out of here but fast!
n.
Idiom: A concern or objection: My offer is final, no ifs, ands, or buts.
but for
Were it not for: except for: We would have reached the summit but for the weather.
[Middle English, from Old English būtan; see ud- in the Appendix of Indo-European roots.]
Usage Note: Should it be No one but I has read it or No one but me has read it? The traditional argument for I is that but is a conjunction in these sentences, coordinating the two parallel subjects of read (“no one” and the speaker), and thus should be followed by the subjective form I. A problem for this analysis is that the objective form me is appropriate when the but phrase occurs at the end of a sentence, as in No one has read it but me. In fact, but is a preposition in both of these constructions. If but were truly a conjunction, the verb would agree in person and number with the noun or pronoun following but, yielding No one but the students have read it, which is clearly wrong. Furthermore, but me in these sentences acts just like a prepositional phrase in being detachable from the rest of its phrase: No one but me has left / No one has left but me is similar to the alternation between John, along with everyone else in the class, left and John left, along with everyone else in the class—both of which contrast with a similar sentence using a conjunction such as and: one can say John and everyone else in the class left, but not John left and everyone else in the class. For these reasons but should be treated as a preposition in these constructions, taking the objective pronouns me and them in all positions. A large majority of the Usage Panel agreed with this analysis in our 2016 survey; only 46 percent accepted No one has read it but I and only 36 accepted No one but I has read it, whereas at least 92 percent accepted the same sentences with me. · But is generally not followed by a comma. Correct written style requires Kim wanted to go, but we stayed, not Kim wanted to go, but, we stayed. · Some people believe that but may not be used to begin a sentence, but in fact this usage is acceptable in all levels of style. See Usage Notes at and, cannot, doubt, however, I1.