Theme

主题，主位；题元

A term used in linguistics as part of an analysis of the structure of sentences (their thematic structure): it refers, not to the subject-matter of a sentence (its everyday meaning), but to the way speakers identify the relative importance of their subject-matter, and is defined as the first major constituent of a sentence (seen here as a string of constituents). There is no necessary correspondence with a functional grammatical element (though in English theme and subject often coincide) e.g. The man is going. The process of moving an element to the front of the sentence in this way (‘fronting’), to act as theme, is known as thematization (sometimes topicalization) or thematic fronting. Some linguists systematically distinguish this notion from other ways of analysing the organization of the sentence structure of messages, such as the topic / comment distinction, or an analysis in terms of information structure.

In the Prague School approach to linguistics, theme is opposed to rheme, producing a distinction similar to that of topic / comment, but interpreted with reference to the theoretical framework of functional sentence perspective. In this theory, the theme is defined as the part of a sentence which adds least to the advancing process of communication (it has the lowest degree of communicative dynamism); in other words, it expresses relatively little (or no) extra meaning, in addition to what has already been communicated. The rheme, by contrast, carries the highest degree of communicative dynamism. Various transitional expressions, neither ‘thematic’ nor ‘rhematic’, are also recognized.

语言学用来分析句子结构（句子的主述结构）的术语；“主题”不是指句子的主要内容（“主题”一词的日常意义），而是指说话人如何表明主要内容的相对重要性，可定义为句子的第一个主要组构成分（句子被视为若干组构成分构成的语符列）。主题与语法功能成分之间不一定有对应关系（虽然英语的主题常与主语重合），例如The man is coming.“那个人就来”。将一个成分移到句首（“前置”）充当主题称作主题化（有时也叫话题化）或主题前置。有的语言学家将这种分析与句子内容结构的其他分析法系统地区别开来，后者如分析为话题/评述，或从信息结构出发的分析。

近年来语言学的布拉格学派用“主位”对立于述位，类似于话题/评述的对立，但需参照功能句法观的理论框架来理解。按这种理论，主位定义为句子中对推进信递过程贡献最小的那个部分（即信递动力作用最小）；换言之，主位除表达已经传递的意义外几乎不（或完全不）表达新的意义。相反，述位所起的信递动力作用最大。“主位”和“述位”之间有各种不同的过渡性词语。