担忧 (dān yōu) means 'to worry, to be concerned' and is more formal and written than the everyday word 担心 (dān xīn). It expresses a sustained, often deeper concern, while 忧虑 (yōu lǜ) emphasizes anxiety and 操心 (cāo xīn) implies taking trouble over someone's affairs. Use 担忧 for serious or long-term worries.