本項研究旨在探討前置的英語時間狀語子句是如何對其主句發揮‘路標’功能的。以往的研究對這一功能的闡釋主要是通過揭示前置時間狀語子句在其主句和前文之間所提供的語義銜接，然而從本項研究所收集的語料來看，前置時間狀語子句和前文並不總是關係密切，這一观察表明，前置時間狀語子句在其主句和前文之間所提供的語義銜接並不能充分反映其前置功能的動因。既然前置時間狀語子句和前文的語義關聯具有不確定性，而它們相對於其主句縂是發揮著一種路標或者鋪墊的功能，本項研究認爲有必要探討前置時間狀語子句與其主句的各種語義關係，并在此基礎上歸納出它們相對於其主句的語義角色。本項研究從這一視角對50篇英文敘事文中前置時間狀語子句的分析顯示，大部分時間狀語子句的功能是作爲一個新事件序列的開頭，其它的功能有提示事件順序的顛倒，指出前一事件序列的完結，描述有關的情勢，提供純粹的時間訊息，或設定主句事件的地點。該分析還以前置時間狀語子句在段落首和段落中的使用情形為根據進一步揭示了這些語義角色的差異。本項研究的結果因而對前置時間狀語子句在不同語境中的產生以及以何種方式對其主句發揮路標功能提供了有用的訊息。

The present study investigates how preposed adverbial clauses of time (ACTs) in English serve as a ‘signpost’ to their main clause. Previous accounts of this function mainly pointed to the thematic link provided by preposed ACTs between their main clause and the preceding discourse. According to the data collected by the present study, however, preposed ACTs may not always be closely related to the preceding discourse, suggesting that the thematic link between preposed ACTs and the preceding discourse does not fully reflect what motivates their usage in the preposed position. Based on the observation that the thematic relationship between preposed ACTs and the preceding discourse may be varied while their role in relation to their main clause remains constant, i.e., as a kind of signpost or grounding link, the present study finds it worthwhile to look into the ways preposed ACTs are thematically related to their main clause and on this basis categorize their thematic roles in relation to their main clause. The analysis it carried out from this perspective of the preposed ACTs taken from 50 different narrative texts shows that the majority of them tend to be used to initiate a new event sequence while the others may be used to signal a reverse temporal relationship, announce the ending of a preceding event sequence, describe part of the circumstances, provide sheer temporal information, or establish a locational setting. The analysis further reveals the different nature of these thematic roles by examining the occurrences of the preposed ACTs in paragraph-internal and paragraph-initial sentences. The results of the study thus provide some useful information as to how preposed ACTs are created in various contexts and in what essential ways they serve as a signpost to their main clause.