Topic and comment
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 opposition of the given/new information is called information structure. That the information structure of a clause is divided in this way is generally agreed on, but the boundary between topic/theme and comment/rheme/focus depends on grammatical theory.
The difference between "topic" and grammatical subject is that topic is used to describe the information structure, or pragmatic structure of a clause and how it coheres with other clauses, whereas the subject is a purely grammatical category. Topic and subject must also be distinguished from actor (or agent), the "doer". 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. 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 distinction 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, František Daneš, Petr Sgall and Eva Hajičová. They have been concerned mainly by its relation to intonation and word-order. Mathesius also pointed 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.
The sentence- or clause-level "topic", or "theme", 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) a special position in a clause (often at the right or left-edge of the clause) where topics typically appear.
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):
- (1) The dog bit the little girl.
- (2) The little girl was bitten by the dog.
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:
- (3) As for the little girl, the dog bit her.
- (4) It was the little girl that the dog bit.
The case of expletives is sometimes rather complex. Consider sentences with expletives (meaningless subjects), like:
- (6) It is raining.
- (7) There is some room in this house.
- (8) There are two days 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 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" (ex. "[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.
In other languages
- Japanese and Korean: the topic is normally marked with a postposition such as -wa (は) or 는/은, -(n)eun.
- In Ivorian French, the topic is marked by the postposition « là ». The topic can be a noun or a nominal group but not necessarily : « Voiture-là est jolie deh » ; « Aujourd'hui-là il fait chaud » ; « Pour toi-là n'est pas comme pour moi hein » ; « Nous qui sommes ici-là, on attend ça seulement ».
- So-called free-word order languages (e.g. Russian, Czech, to a certain extent Chinese and German) use word-order as the primary means. Usually the 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 follow its comment. In this case, the syntactic subject of the sentence is an expletive זה ("ze", lit. "this"). 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".
- 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.
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 the information retrieval  and the automatic summarization .
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