额外 (éwài) is an adjective meaning 'extra, additional', used for something beyond the standard or required amount (e.g., extra costs, extra work). It directly modifies nouns (额外的工作). In contrast, 另外 is primarily an adverb or pronoun meaning 'in addition' or 'another'; 附带 means 'attached, supplementary' and often implies something incidental that comes along with the main item.