Hello daagh
I'm a big fan of your explanations, you are just amazing, and thank you for being active again in this forum.

the word no one is playing a crucial role in deciding whether but... is conjunction or preposition
Suppose if we have every one instead of no one, I think but cannot work as conjunction as you've mentioned earlier.

Every one but him could have told them that the thief was I.
The meaning of the above sentence is every one could have told them but he couldn't.
Here but... -- meaning apart from -- is a preposition and the noun phrase every one but him... acts as subject.


But if but... acts as conjunction and the pronoun him is he, then it will distort the intended meaning of the sentence.
but he could have told them that the thief was I
-->This sentence means that he could have done the act of telling them but in fact he couldn't.

Thangvietnam
Hi

According to Macmillan dictionary online

but is conjunction when it connects to two clauses
but is a preposition when it connect two nouns

If you read the dictionary correctly, ‘but’ as a preposition joins two nouns but not two object pronouns or a noun and an object pronoun such as ’him’

Can we say ‘him but me is going to the movie? We can’t.

No one but him could have told them that the thief was I

In this case too, ‘him’ is not a nominative noun but an object pronoun.

So it has to be a nominative pronoun 'he' after but
_________________

Kani

Since you're a stickler for rules, the following are comprehensive lessons on pronouns,
which I happened to found in the book "When Bad Grammar Happens to Good People"

(1) Use the subjective case when…
Rule #1a: The pronoun is the subject of the verb.
e.g.: I, we, you, he, she, it, they ran away.
Rule #1b: The pronoun follows a finite form of the verb “to be”
(any form of “to be” except the infinitive form with the
“to” in front of it).
e.g.: They believed that the thief was I, you, he, she.
They believed that the thieves were we, you, they.

(2) Use the objective case when…
Rule #2a: The pronoun is the object of a verb.
e.g.: The search team found me, him, her, it, us, you, them.
Rule #2b: The pronoun is the indirect object of a verb.
e.g.: Bob gave me, him, her, us, you, them all the zucchini
in his garden.
Rule #2c: The pronoun is the subject of an infinitive.
e.g. The boss told me, him, her, us, you, them to do it.
Rule #2d: The pronoun is the object of an infinitive.
e.g.: The judge wanted to believe me, him, her, it, us, you, them.
Rule #2e: The pronoun is the object of a preposition.
e.g.: Put the blanket over me, you, him, her, it, us, them.

(3) Use the possessive case when…
Rule #3a: The pronoun itself is being used to indicate possession.
e.g.: My, your, her, his, our, their toaster.

Moreover, You assumed that every word in English would've only one function or use.
i.e. every word belongs to one class of Parts of Speech (there are eight of them).
For instance, you thought the word "but" is a preposition, but in the examples you cited "but" acts a conjunction.
Also, the word "as" acts a subordinating conjunction by introducing the basis of comparison.

To know more about how the same word can be used as different parts of speech,
please buy a good grammar and learn the basics.

For such questions I practice a simple method- ask your self questions?

Who could say? Of course you will say- "he could say" or if the questions is "Who has done this?- of course you will say "he has done this/that?

So, try to imagine this sentence as- " I know that I (not me) am the thief and only he (not him) could say/know that".

Also, try saying " me was the thief" & "I was the thief"? Which one of them sounds better? of course the second sounds better and first makes almost no sense. I have noticed that apart from grammar logic these questions can also be solved with the help of asking yourself few questions.



Regards
Yash

Hi daagh

You mention that : "hence, no one but he" is the correct version but you chose B for an answer

Furthermore, In the second part of the sentence, shouldn't "Me" , the object pronoun of "I", be more appropriate as the usage is as an object.

thanks

pretty clear. thanks