This paper studied the semantic and syntactic functions of the modern Chinese adverb ‘YOU’. The original meaning and basic meaning of the adverb ‘YOU’ is ‘addition’, and the meaning of ‘time order’, ‘repeat’, ‘continue’, ‘intensify’, ‘return’ of ‘YOU’ is based on the meaning of ‘addition’. And It was revealed that ‘subject/topic’, ‘whether the predicate phrase is the same’, ‘type of predicate phrase’, and ‘temporality’ are factors involved in the formation of the meaning of the adverb ‘YOU’ and the difference between the meanings. In addition, ‘YOU’ can function as a connecting adverb that connects words, phrases, clauses, sentences, and paragraphs. And, although the word order of the adverb ‘YOU’ is always in front of the predicate, the word order of the auxiliary verb and the adverb ‘YOU’ and the word order of the negative and the adverb ‘YOU’ may vary depending on the meaning range encompassed by ‘YOU’.