"but" usage (redundancy of "but")

Question

We all know that "but" is used to replace "except" or indicate that the first clause is contrastive to the second in a way, or the logic these two sentences bear is somewhat contradictory. But, I see that there is also a redundant use of "but". I want to confirm whether it is a form of redundancy, or it is just me. Please consider the following:

1. I am not a teacher, BUT I am a student.
2. It is not a gun, BUT it is a curtain.
3. He is not a president, BUT he is a clown.

Answer 1

I think you answered your own question!

"but" is used to […] indicate that the first clause is contrastive to the second in a way

In your first example, "I am not a teacher" contrasts with "I am a student". In context, I'd imagine something like this:

Alice, in a school: Excuse me, are you a teacher? I have some questions about the organization here.
Bob: I'm not a teacher… [and thus might not be able to answer]
Bob: …but I am a student. [and thus can answer]

In other words, the "but" is because the implication of the first part is the opposite of the implication of the second part.

In your other two examples, I wouldn't use "but" in most cases: only in a context where the two clauses had different implicatures (like above).

Answer 2

The examples you are given are not natural, although maybe formally correct. Colloquially, you'll rather find

I am not a teacher, but a student

where but is adverbially linking student to the verb of the matrix clause We can readily see that I am not a student, [and] a teacher is ambiguous; different bracketing yields different ways to parse the negation; If anything, the intended negation is ungrammatical because the number of the singular subject disagrees with the two-headed object. Redundancy is thus required to disambiguate, e.g: I am not a student, not a teacher, which however is equally unlikely, and rather said using neither, nor. The base case I am a student, [and] a teacher [and a unicorn] is still ambiguous due to the semantics, where teacher and student are usually in opposition, so as well as might be used as the conjunction of choice to mark simultanity. Copula are regularly required between two indefinite nouns, except for adpositions (a teacher, a smart one).

The only thing that's redundant in your example is the repeated verb. If amn't is parsed as a lexeme, then I am not X, I am Y is anaphoric without redundancy. However, that's rarely the case, as shown in the frequent reversal I am a Y, not an X.

There's another reason to use the conjunction: *It is not the show it ..." rather expects "... used to be", a syntax that can be explicitly disambiguited with the conjunction that. It seems reasonable to assume the indefinite article would follow the same pattern of the definite article, for simplicity of the paradigm, even if a definite pronoun it cannot be in adposition to an indefinite noun (rather it needs that, which then functions as a determiner: It is not a show that is shown on TV). Moving out on a limb I guess we may still see It will not rain, it won't, which is prefectly redundant, that is seen all too frequently in reversed order in anaphoric questions, isn't it?

Answer 3

The examples that you picked are where but is simply being a conjunction joining 2 sentences/clauses. The usage of but there seems redundant for that reason but (note the meta usage of the very word here) without it you'll have 2 separate sentences. So you'd have to say

I am not a teacher. I am a student

or with the conjunction

I am not a teacher but a student

There is still a possibility for a different question. Perhaps a parent looking for some employee of the school for some help with directions could ask, "Are you a teacher?"

I am not a teacher but I could help.

Note, the second part is not even contrasting the first. Again, without the but

I am not a teacher. I could help.

It is still not redundancy much like other conjunctions aren't.