In linguistics, the topic, or theme, of a sentence is what is being talked about, and the comment (rheme or focus) is what is being said about the topic. This division into old vs. new content is called information structure. It is generally agreed that clauses are divided into topic vs. comment, but in certain cases the boundary between them depends on which specific grammatical theory is being used to analyze the sentence.
Topic, which is defined by pragmatic considerations, is a distinct concept from grammatical subject, which is defined by syntax. In any given sentence these may be the same, but they need not be. For example, in the sentence "As for the little girl, the dog bit her", the subject is "the dog" but the topic is "the little girl".
Topic and subject are also distinct concepts from agent (or actor)--the "doer", which is defined by semantics. In English clauses with a verb in the passive voice, for instance, the topic is typically the subject, while the agent may be omitted or may follow the preposition by. For example, in the sentence "The little girl was bitten by the dog", "the little girl" is the subject and the topic, but "the dog" is the agent.
In some languages, word order and other syntactic phenomena are determined largely by the topic-comment (theme-rheme) structure. These languages are sometimes referred to as topic-prominent languages. Korean and Japanese are often given as examples of this.
The sentence- or clause-level "topic", or "theme", can be defined in a number of different ways. Among the most common are
In an ordinary English clause, the subject is normally the same as the topic/theme (example 1), even in the passive voice (where the subject is a patient, not an agent: example 2):
These clauses have different topics: the first is about the dog, and the second about the little girl.
In English it is also possible to use other sentence structures to show the topic of the sentence, as in the following:
The case of expletives is sometimes rather complex. Consider sentences with expletives (meaningless subjects), like:
In these examples the syntactic subject position (to the left of the verb) is manned by the meaningless expletive ("it" or "there"), whose sole purpose is satisfying the extended projection principle, and is nevertheless necessary. In these sentences the topic is never the subject, but is determined pragmatically. In all these cases, the whole sentence refers to the comment part.
The relation between topic/theme and comment/rheme/focus should not be confused with the topic-comment relation in Rhetorical Structure Theory-Discourse Treebank (RST-DT corpus) where it is defined as "a general statement or topic of discussion is introduced, after which a specific remark is made on the statement or topic". For example: "[As far as the pound goes,] [some traders say a slide toward support at 1.5500 may be a favorable development for the dollar this week.]"
Different languages mark topics in different ways. Distinct intonation and word-order are the most common means. The tendency to place topicalized constituents sentence-initially ("topic fronting") is widespread. Topic fronting refers to placing the topic at the beginning of a clause regardless whether it is marked or not. Again, linguists disagree on many details.
Languages often show different kinds of grammar for sentences that introduce new topics and those that continue discussing previously established topics.
When a sentence continues discussing a previously established topic, it is likely to use pronouns to refer to the topic. Such topics tend to be subjects. In many languages, pronouns referring to previously established topics will show pro-drop.
The topic/theme comes first in the clause, and is typically marked out by intonation as well.
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The main application of the topic-comment structure is in the domain of speech technology, especially the design of embodied conversational agents (intonational focus assignment, relation between information structure and posture and gesture). There were some attempts to apply the theory of topic/comment for information retrieval  and automatic summarization.
The distinction between subject and topic was probably first suggested by Henri Weil in 1844. He established the connection between information structure and word order. Georg von der Gabelentz distinguished psychological subject (roughly topic) and psychological object (roughly focus). In the Prague school, the dichotomy, termed topic-focus articulation, has been studied mainly by Vilém Mathesius,Jan Firbas, Franti?ek Dane?, Petr Sgall and Eva Haji?ová. They have been concerned mainly by its relation to intonation and word-order. Mathesius also pointed out that the topic does not provide new information but connects the sentence to the context. The work of Michael Halliday in the 1960s is responsible for developing linguistic science through his systemic functional linguistics model for English.
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