同义词辨析的教学一直是是对外汉语教学中举足轻重的环节，有关同义词的辨析也是不胜枚举。“必然”和“必定”是汉语同义词中非常有代表性的词语，无论在语义上还是语用上都呈现出极高的相似性。“必然”和“必定”是表示确定的副词，两个词都可以在句中做状语，也都可以修饰动词和形容词。但是“必然”还可以作名词和形容词，两者在教学中的差别还是不容小觑的。外国学习者往往会因为细微的差别产生偏误。本文对两个词语进行分析研究，找出偏误及原因，并根据偏误给出相关的教学建议。

The teaching of synonym analysis has always been an important link in teaching Chinese as a foreign language, and there are numerous papers on synonym analysis. "Bi ran" and "Bi ding" are very representative words in Chinese synonyms, which show high similarity both in semantics and pragmatics. The differences between them in teaching should not be ignored, and foreign learners often make mistakes because of the subtle differences. This paper makes a semantic, syntactic and pragmatic analysis of the two words, finds out the types of errors and the causes of the errors, and according to the errors, puts forward the proper attitude towards the errors, as well as the teaching and learning countermeasures for teachers and students.