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 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.

The topic of a sentence is distinct from the grammatical subject. The topic is defined by pragmatic considerations, that is, the context that provides meaning. The grammatical subject is defined by syntax. In any given sentence the topic and grammatical subject 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 being what is being talked about and the subject being what is doing the action can, also, be distinct concepts from the concept agent (or actor)—the "doer", which is defined by semantics, that is, by the contextual meaning of the sentence in the paragraph. 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.