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.