TrevorKleeTutor wrote:
The reason why it's "I am looking for a tutor who understands..." is that you have a phrase in which "who" is serving as the subject. So, your phrase is, "I am looking for a tutor /who understands every in and out of the GMAT verbal/". Take that on its own, switch out the pronoun, and you get "He understands every in and out of the GMAT verbal", which is an obvious subject pronoun. You just have to make sure you know what purpose the pronoun is serving in the sentence.
In my opinion, "who" is not serving as a subject in this instance. I am looking for a tutor, hence "I" is the subject of the sentence and "tutor" is the object. "Who" simply describes the tutor. That which describes/modifies a noun is technically an adjective, not a noun, and an adjective cannot serve as a subject.
The dead giveaway, as mentioned above, is that "tutor" is preceded by a preposition ("for"). Since "who" further describes the tutor being looked for, "who" is serving neither as the subject nor the object, but instead as a
modifier to the object, in the same way that the word "that" would if we were describing a thing rather than a person. In this example, "who" is serving an adjective (a.k.a. modifier) instead of a noun.
"That" is a similar word, in the sense that is can be used both as a noun ("That which interests me usually involves struggle") as well as an adjective ("I enjoy puzzles that are challenging"). However, you wouldn't say "I enjoy puzzles they are challenging," because that would be two sentences, not one. And you wouldn't say "I am looking for a tutor he understands" for the same reason.
It is possible to have more than one subject in a sentence, but in order to have a second subject, you would have to insert some sort of conjunction or punctuation to break it into parts, such as "unless" or "and": "I don't enjoy puzzles
unless they are challenging" or "I am looking for a tutor,
and I hope that he understands every in and out of the GMAT verbal."