Fun fact: In Spanish, all the capitalization rules are different--you don't capitalize anything except the first word of the title of a novel or poem plus any proper nouns. (And a few days before the AP Lit test is 100% the right time to learn this
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I learned capitalization long before any class taught it, with the basic rule of "little words don't get capital letters", by reading a lot, and possibly my mom telling me. (I have this dim memory of being very small, at Costco in the parking lot, reading something and thinking or maybe asking about it.) Grammar tends to be taught in elementary school and then forgotten until higher-level English in high school, which is sometimes too late, in my experience. (U.S. public schools, California Bay Area)