Which is the subject in the phrase "I always believed I was the only one who could make you happy."? Is it "I"? Is it "the only one"? or is it some other phrase or word?
Which is the subject in the phrase
It's not a phrase; it's a sentence. Phrases are parts of sentences. So are subordinate clauses.
This sentence has three clauses, in descending order. Each one has a subject.
- I always believed I was the only one who could make you happy.
- [I always believed [(that) I was the only one [who could make you happy.]]]
- [I always believed [(that)
(2)]] (main clause, subject I)
- [I was the only one [
(3)]] (object clause of believe, subject I)
- [who could make you happy] (relative clause modifying one, subject who)
As you can see, there isn't any "the subject" for a complex sentence. There is a subject for every clause in a sentence, and there is a verb in every clause, too; in fact, that's what makes it a clause and not a phrase -- subject and verb.
I may be wrong, but I think that here we have a complex sentence. It has one main sentence and two clauses. The subject of the main part is "I", as well as the subject of the first clause is also "I". And the second clause has the subject "who" (I think so). I don't think "the only one" is a subject. It can be a predicate though.