I've seen this sentence in someone's facebook status but it sounds unnatural to me.
I'm the Rumplestiltskin who spins straw into gold
I thought it should be spin because the subject is I, but I'm not sure. Which one is the subject in this sentence, I or Rumplestiltskin and why?
The subject of the main clause is I. But then that subject talks about himself in the third person: The Rumplestiltskin who spins straw into gold. Spins agrees with the subject of the relative clause.
This is one sentence, but there are two clauses. This means that there are two subjects, and each of those subjects is paired with its own verb.
Let's give descriptions to the two clauses. One is a matrix clause. That's a good description of a clause that contains another clause. The other is a subordinate clause. That's a good description of a clause that serves a function within another clause.
The subject of the matrix clause is "I". The verb of the matrix clause is "am". In your example, subject and verb are contracted, yielding "I'm". The "am" agrees with the first-person singular subject. The rest of the sentence is the verb's argument, in this case a subjective complement.
The subject of the subordinate clause is "who". This kind of subordinate clause is called a relative clause. The relative pronoun "who" relates this clause to the proper noun "Rumplestiltskin". The relative pronoun borrows the person and number of its antecedent, and so this "who" is the same third-person singular as "Rumplestiltskin". The verb of the subordinate clause is "spins". "Spins" agrees with a third-person singular subject.
These relationships become more obvious if we split the original sentence into two independent clauses:
Rumplestiltskin spins straw into gold, and I'm Rumplestiltskin.
One possible point of confusion is that the subject of the matrix clause does not need to agree with the subjective complement, in either person or number. In fact, first-person subjects are usually paired with third-person complements.
Another possible point of confusion is that we often say "the subject of the sentence" when, in fact, there is no such thing. A clause has a subject. A complete sentence has at least one clause. Often, a sentence contains exactly one independent clause, and, in this case, "the subject of the sentence" is convenient shorthand for "the subject of the only clause in the sentence". That shorthand can be both inconvenient and misleading whenever we encounter a compound or complex sentence.
There are two subject/verb pairings in the original, complex sentence. There are still two subject/verb pairings in my paraphrased compound sentence. In neither case does it make any sense to talk about (or even think about) the one-and-only subject of a sentence with two subjects.
The subject is Rumplestiltskin. The "I'm" means "I am," which modifies what exactly the subject is. The verb, spins, is not spun by I. It is spun by Rumplestiltskin.
