Everyone assumes Brittany is passing Spanish because Santana does her work. What they don't know is that Brittany has been fluent since she was 5.
She still remembers the day that they got a new girl in kindergarten, a tiny dark-haired girl who spoke with an accent and hit anyone who made fun of her for it. But when their moms came to pick them up, Brittany heard Santana talking to her mom in the most beautiful language she had ever heard. Brittany's mom told her they were speaking Spanish. The next week Brittany started taking Spanish lessons at the community center because she had begged her mom nonstop until she gave in.
The few people who know this much of the story assume that Brittany wanted to learn Spanish because of her sunny, welcoming demeanor and that she just wanted to make Santana feel comfortable or maybe just because she liked the language.
But the real reason she wanted to learn is because, even at 5, Santana was the most beautiful thing she had ever seen and she wanted to tell her, but she knew no words in English could possibly do her justice.