Mulțumesc is the standard way to say 'thank you' in Romanian and works in both spoken and written contexts. Use 'mersi' or 'merci' in informal situations. For a more formal or polite form, say 'vă mulțumesc' (to one person formally or to several people). Common extensions: 'mulțumesc mult' (thank you very much) or 'mulțumesc frumos' (thank you kindly).