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.
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.
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 necessary.
[1] 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".
For example: "[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.
In many languages, pronouns referring to previously established topics will show pro-drop.
But whereas topic-prominent languages might use this approach by default or obligately, in subject-prominent ones such as English it is merely an option that often is not invoked.
In the Prague school, the dichotomy, termed topic–focus articulation, has been studied mainly by Vilém Mathesius,[10] Jan Firbas, František Daneš, Petr Sgall and Eva Hajičová.
Mathesius also pointed out that the topic does not provide new information but connects the sentence to the context.