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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
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:
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:
A distinction must be made between the clause-level topic and the discourse-level topic. Suppose we are talking about Mike's house:
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:
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).
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.
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.
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