小心 (xiǎo xīn) is the most common and versatile word for 'be careful' in Chinese. As a verb, it warns of potential danger (like 当心 and 留神, but more general); as an adjective, it describes a cautious person. The adjectives 仔细, 细心, 谨慎, and 用心 each add a specific nuance: attention to detail, thoroughness, prudence, or concentration.