弗 (fú) is a classical Chinese negation adverb meaning 'not', equivalent to 不 (bù) but much more formal and literary. It appears primarily in classical texts, proverbs, formal writing, and set phrases. You don't use it in everyday spoken Mandarin — 不 is the standard choice. Recognizing 弗 is important for reading classical Chinese and for understanding idioms like 不学弗知 ('not learning, you won't know').