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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 of the information structure of the sentence is generally agreed upon, but further than this the definition of what constitutes a "topic" depends on which particular grammatical theory is being employed.
The difference between "topic" and grammatical subject is that topic is used to describe the information structure, or pragmatic structure of a sentence and how it coheres with other sentences, whereas the subject is a purely grammatical category. For example it is possible to have sentences where the subject is not the topic, for example often the case in passive sentences. In some languages word order and other syntactic phenomena is determined largely by the topic-comment structure, rather than by the grammatical structure of the sentence. These languages are sometimes referred to as topic-prominent languages. Chinese is often given as an example of this.
The distinction was probably first suggested by Henri Weil in 1844. 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, František Daneš, Petr Sgall and Eva Hajičová. They have been concerned mainly by its relation to intonation and word-order. The work of Michael Halliday in the 1960s is likely responsible for bringing the ideas to functional grammar.
Note that in some categorizations, topic refers only to the contrastive theme and comment to the noncontrastive theme + rheme.
The term "topic" can be defined in a number of different ways. Among the most common are
- a.) the phrase in a clause that the rest of the clause is understood to be about,
- b.) the phrase in a discourse that the rest of the discourse is understood to be about,
- c.) a special position in a clause (often at the right or left-edge of the clause) where topics typically appear.
In an ordinary English sentence, the subject is normally the same as the topic. For example, the topic is emphasized in italics in the following sentences:
- (1) The dog bit the little girl.
- (2) The little girl was bitten by the dog.
Although these sentences mean the same thing, they have different topics. The first sentence is about the dog, while the second is 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:
- (3) As for the little girl, the dog bit her.
A distinction must be made between the clause-level topic and the discourse-level topic. Suppose we are talking about Mike's house:
- (4) Mike's house was very comfortable and warm! He really didn't want to leave, but he couldn't afford the rent, you know. And it had such a nice garden in the back!
In the example, the discourse-level topic is established in the first sentence: it is Mike's house. In the following sentence, a new "local" topic is established on the sentence level: he (Mike). But the discourse-level topic is still Mike's house, which is why the last comment does not seem out of place.
The case of expletives best examplifies the subject-topic distinction. Consider sentences with expletives (meaningless subjects), like:
- (5) It is raining.
- (6) There is some room in this house.
- (7) There is a day in the year in which the day and the night are equal in length.
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 unnecessary. In these sentences the topic is never the subject, but is determined pragmatically. In (5) the topic is the whole proposition expressed by the sentence (i.e, the fact that it is raining). In (6) it is "some room". In (7) it is arguably the equality in length of the day and night in some day (rather than the day itself).
Realization of topic–comment 
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. Again, linguists disagree on many details.
- English: intonation is the primary means, although word order (e.g., fronting of contrasted topics: Kim, I like.) and other syntactic (passivisation, clefting) or lexical means ("As for ...", "Regarding ...") are also employed.
- Japanese and Korean: topic is often marked with a special clitic postposition such as (は, -wa; 는/은, -(n)un).
- So called free-word order languages (e.g. Russian, Czech, to a certain extent Chinese or German) use word-order as the primary means. Usually topic precedes focus. However, for example in Czech, both orders are possible. The order with comment sentence-initial is referred as subjective (Vilém Mathesius invented the term and opposed it to objective) and expresses certain emotional involvement. The two orders are distinguished by intonation.
- In Modern Hebrew, a topic may be adjoined to a sentence from the right hand side, while the syntactic subject of the sentence is an expletive. For example, זה מאד מענין הספר הזה "ze meod meanyen ha-sefer ha-ze" (lit. "This is very interesting this book") means "This book is very interesting". The syntactic subject is "ze", which is meaningless, while the topic is "ha-sefer ha-ze" ("this book"), which appears to the right of (i.e. after the main clause of) the sentence, and not in its canonical subject position (which is occupied by the meaningless "ze"). (Note that "to the right" speaks to the phrase's location in the standard linguistic representation of the sentence – left-to-right, Roman Alphabet – and is independent of the directionality of the given language's native script; hence the topic phrase is said to appear to the right, even though in Hebrew writing it is seen on the left.)
- In American Sign Language, a topic can be declared at the beginning of a sentence (indicated by raised eyebrows and head tilt) describing the object, then the rest of the sentence describes what happens to that object.
Languages often show different kinds of grammar for sentences that introduce new topics and those that continue discussing previously established topics. When a sentence introduces a new topic for discussion, it is most likely to use one of the strategies mentioned in (b), or (c) above.
When a sentence continues discussing a previously established topic, it is likely to use pronouns to refer to the topic. Topics of this sort show a tendency to be subjects, as mentioned in (a) above. In many languages, pronouns referring to previously established topics will show pro-drop.
See also 
- Givón, Talmy. 1983a. Topic continuity in discourse: A quantitative cross-language study. Amsterdam: Arshdeep Singh.
- Hajičová, Eva, Partee, Barbara H., Sgall, Petr. 1998. Topic–Focus Articulation, Tripartite Structures, and Semantic Content. Studies in Linguistics and Philosophy 71. Dordrecht: Kluwer. (ix + 216 pp.) review
- Hockett, Charles F.. 1958. A Course in Modern Linguistics. New York: The Macmillan Company. (pp. 191–208)
- Mathesius, Vilém. 1975. A Functional Analysis of Present Day English on a General Linguistic Basis. edited by Josef Vachek, translated by Libuše Dušková. The Hague – Paris: Mouton.
- Kadmon, Nirit. 2001. Pragmatics Blackwell Publishers. Blackwell Publishers.
- Lambrecht, Knud. 1994. Information structure and sentence form. Cambridge: Cambridge University Press.
- Li, Charles N., Thompson, Sandra A. 1976. Subject and Topic: A New Typology of Languages, in: Li, Charles N. (ed.) Subject and Topic, New York/San Francisco/London: Academic Press, 457–490.
- Payne, Thomas E. 1997. Describing morphosyntax: A guide for field linguists. Cambridge: Cambridge University Press.
- Von der Gabelentz, Georg. 1891. Die Sprachwissenschaft, ihre Aufgaben, Methoden und bisherigen Ergebnisse. Leipzig: T.O. Weigel Nachfolger.
- Weil, Henri. 1887. De l'ordre des mots dans les langues anciennes comparées aux langues modernes: question de grammaire générale. 1844. Published in English as The order of words in the ancient languages compared with that of the modern languages.