In linguistics, the topic (or theme) is the part of the proposition that is being talked about (predicated). The topic is therefore "old news", i. e. the things already mentioned and understood. For example, the topic is emphasized like this in the following sentences:
- The dog bit the little girl.
- The little girl was bitten by the dog.
- It was the little girl that the dog bit.
- The little girl, the dog bit her.
A distinction must be made between the sentence-level topic and the discourse-level topic. If we are talking about Mike's house being comfortable and warm, and then we say "He didn't want to leave", the sentence-level topic is "he" (that is, Mike), but the discourse-level topic is still his home.
Many languages, like English, resort to different means in order to signal a new topic, such as:
- Stating it explicitly as the subject (which tends to be considered more topic-like by the speakers).
- Using passive voice to transform an object into a subject (for the above reason).
- Emphasizing the topic using clefting.
- Through periphrastic constructions like "As for...", "Speaking of...", etc.
- Using left dislocation (called topic fronting, i. e. moving the topic to the beginning of the sentence).
There are some other languages, like Japanese, that work directly on a topic-comment frame. A new topic is always introduced in a specific way (Japanese uses a postposition, wa), and then it is left out and implicit throughout the discourse, until a new topic appears. The topic can be the subject or the object of a verb, but it can also be an indirect object or even an oblique complement of any kind. It is always dislocated to the front of the sentence.
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