Why was the only subject in school that Holden actually passed without much trouble English?
Does this hold some sort of connection with his isolation from society and him somehow using English as a way to escape from the world? I never understood it; if he was mentally unstable and was passing English, what REALLY stopped him from not putting any application towards other subjects?
Early on, Holden tells Mr. Spencer that he passed English because he had the same class the year before, when he was at The Whooton School, so he did not have to work very hard, just "write compositions once in awhile."
Nevertheless, Holden is also very good in English. He is well-read, often quoting classics, such as Romeo and Juliet, The Return of the Native, and Out of Africa. The reader also gets to see his creative side, when he writes the descriptive piece for Stradlater, which he chooses to write on Allie's baseball glove. We understand his thought process better at this point, but we also understand from where his instability stems. It is interesting to note that he writes this piece in order to escape the world. He could not stop thinking about Stradlater and what he might be doing on his date with Jane, so he writes the essay, thus thinking about something else.
There is no doubt that Holden is smart. He puts little effort into his work, probably for a number of reasons, including his hatred of the hypocrisy he sees in the school, his fear of growing up, and his inability to accept his brother's death.
Holden is very intelligent, which is part of his problem: he's too smart for high school and can too easily see through people. He's also a compassionate person, so what he perceives as the "phoniness" in others troubles him. His instinct is also to protect other people, especially those he likes or sees as vulnerable. In the opening chapter, as he talks to his history teacher Mr. Spencer, he doesn't want Mr. Spencer to feel bad or to feel that he failed somehow as a teacher because Holden had no interest in his subject. So Holden downplays his own intelligence:
I didn't want to hurt his feelings. He was mad [crazy] about history. ... Well, you could see he really felt pretty lousy about flunking me. So I shot the bull for a while. I told him I was a real moron, and all that stuff.
But in fact, Holden's intelligence and especially his talent in writing and love for English thread throughout the novel. Holden is being disingenuous in the first chapter when he dismisses passing English as a result of the class being easy: that surely helped, but he is also genuinely engaged in the subject.
For instance, his roommate, Stradlater, asks Holden to write an English composition for him because "that sonuvabitch Hartzell thinks you're a hot-shot in English." Holden then gets irritated at Stradlater for thinking good writing is merely about getting the commas in the right places and not about the content of what one writes.
Later, when he meets the nuns, one of whom teaches English, he speaks at length about Romeo and Juliet in a way that shows he is deeply engaged in the play, even if it's not his favorite. He comments on the conversation: "I said I'd enjoyed talking to them a lot, too. I meant it, too."
Holden's love of English is part of who he is and one could argue that it arises, at least in part, out of his frustration with the world around him and his desire to escape into an alternative world--for instance he talks about how he can think about ducks and carry on a conversation at the same time as he talks to Mr. Spencer--but he does genuinely seem to love the subject as well.