“必然”“必定”“一定”都是表示确定性推测的语气副词，在语义及用法上既有联系又有区别。本文在现有研究的基础上，结合大量语料，从语义、句法、语用、语篇等方面对三个词进行了较为全面的比较分析，并探究了造成三者差异的主要原因。
语义方面，本文在指出三个词共性的同时，归纳了三个词在主客观程度、表必然性强弱及对已然/未然情况的推测方面的差异。“必然”“必定”“一定”都可以表示确定性推测，但在主观性程度上“一定＞必定＞必然”，在表必然性的强弱上“必然＞必定＞一定”。三个词都可以用于对未然情况的推测，其中“一定”还可以用于对已然情况的推测。
句法方面，我们考察了三个词的句法位置、否定形式以及与人称代词、能愿动词、判断动词“是”、语气助词的搭配情况。三个词中“必然”“必定”的句法位置较为受限，而“一定”的句法位置比较灵活，有时甚至还可以单独使用。在否定形式上，“必然”“一定”的否定形式是“不必然”“不一定”，而“必定”的否定形式则是“未必”。由于三个词的语义特点不尽相同，它们与其他词语的搭配情况也呈现出不同的特点。
语用方面，我们从句类分布、适用语体、语用功能三个方面来比较三个词的语用表达。从句类分布来看，“必然”“必定”“一定”都只能用于陈述句、感叹句和疑问句。从适用语体来看，“必然”常用于书面语体，“必定”“一定”既可以用于书面语体，也可以用于口语语体，但“必定”更偏向于书面语体，而“一定”更偏向于口语语体。从语用功能来看，三个词都具有高位表述及焦点表述功能、肯定评价及传信评价功能、正向强调及顺接强调功能，“必定”“一定”在用于对话时还具有一定的言语行为功能。
语篇方面，我们考察了三个词在复句以及报刊类篇章中的分布。“必然”“必定”“一定”都常出现在条件复句、因果复句及转折复句中，受主观性程度影响，“一定”的复句分布特点最为特殊。在报刊类篇章的分布上，三个词都可以用于报刊类文章的标题、段落首句或末句中，其中“一定”出现在标题及段落末句中的频率最高，且常用于末段末句。

“Biran”, “Biding” and “Yiding” are all modal adverbs expressing deterministic speculation, which are related and different in meaning and usage. Based on the existing research and a large amount of corpus, this paper makes a comprehensive comparative analysis of the three words from the aspects of semantics, syntax, pragmatics and discourse, and explores the main reasons for the differences between the three words.
In terms of semantics, while pointing out the commonness of the three words, this paper summarizes the differences of the three words in subjective and objective degree, the strength of the necessity of expression and the speculation of what has already happened or not yet. “Biran”, “Biding” and “Yiding” can all represent deterministic speculation, but “Yiding > Biding > Biran” in the degree of subjectivity, and “Biran > Biding > Yiding” in the degree of necessity. All three words can be used to speculate what is to happen, and “Yiding” can also be used to speculate what has already happened.
In the aspect of syntax, we investigate the syntactic position, negative form and collocation with personal pronoun, willing verb, judgment verb “Shi” and modal particle. The syntactic position of “Biran” and “Biding” in the three words is relatively limited, while the syntactic position of “Yiding” is relatively flexible, and sometimes can even be used alone. In negative form, the negative forms of “Biran” and “Yiding” are “Bubiran” and “Buyiding”, while the negative form of “Biding” is “Weibi”. Because the semantic characteristics of the three words are not the same, their collocation with other parts of speech also presents different characteristics.
In terms of pragmatics, we compare the pragmatic expressions of the three words from three aspects: the distribution of sentence types, the applicable language style and the pragmatic function. From the perspective of the distribution of sentence types, “Biran”, “Biding” and “Yiding” can only be used in statements, imperative sentences and questions. From the perspective of the applicable language style, “Biran” is often used in written style, “Biding” and “Yiding” can be used in both written style and spoken style, but “Biding” is more inclined to written style, while “Yiding” is more inclined to spoken style. From the perspective of pragmatic functions, the three words all have the functions of high-level utterance and focus utterance, positive evaluation and evidential evaluation, positive emphasis and sequential emphasis. “Biding” and “Yiding” also have certain speech act functions when used in dialogues.
In terms of discourse, we examine the distribution of three words in complex sentences and newspaper articles. “Biran”, “Biding” and “Yiding” often appear in conditional complex sentences, causal complex sentences and transitional complex sentences. Influenced by the degree of subjectivity, the distribution characteristics of “Yiding” are the most special. In terms of the distribution of newspaper articles, all three words can be used in the title, the first sentence or the last sentence of the paragraph, among which “Yiding” appears in the title and the last sentence of the paragraph with the highest frequency and is often used in the last sentence of the last paragraph.