Although no one ever taught it to you, odds are that if you solve a lot of crossword puzzles, you’re fluent in the grammar of crosswords. Most crossword enthusiasts could explain that nouns clue nouns, verbs clue verbs, and so on. They also come to know—subconsciously—that answers must be interchangeable with their clues in a sentence, even for categories too particular to have a name.
These unspoken tenets can be deceptively complex. Consider how GALORE could be clued by “aplenty,” but not by “many.” This is because, even though you might call them all adjectives, only galore and aplenty come after the noun they modify (whereas most other English adjectives come before). Additionally, simply by filling in enough puzzles, our brains can learn that multi-word crossword answers must form what linguists call a syntactic constituent—a group of words that functions together as a complete unit, the way "safe and reliable" does but "safe and" doesn’t.