选用有标记主位与句子需表达的某种修辞效果有关。

The use of a marked theme, which is selective, results from its certain rhetorical effects.

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在小句层面，处所介词短语把时间处所前景化为有标记主位，而把空间处所背景化。

At the clause stratum, a locative prepositional phrase functions to foreground the temporal location as a marked theme while it functions to background the spatial location.

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有标记主位出现率也不低，为语篇增添了节奏感和吸引力，并增强了句子之间的连贯性。

But a certain number of marked themes are necessary, which add the sense of rhyme and coherence between sentences.

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