It's probably the most common class of error for people who learn English as adults. There's lots of strange, hard to explain meaning encoded in which (if any)

It's probably the most common class of error for people who learn English as adults. There's lots of strange, hard to explain meaning encoded in which (if any) article a person chooses. "I dropped by the school today" and "I dropped by school today" are both valid and have slightly different implications