English language learners may be confused in identifying the grammatical category of such conjunctive expressions as "before," "after" and "since" introducing non-finite "-ing" clauses. In this article, we will conduct a corpus-based investigation of hypotactic conjunctions and conjunctive prepositions following the principle of unidirectional transfer in grammatical metaphor proposed by He and Yang (2014) within the framework of Systemic Functional Linguistics. The research concludes that hypotactic conjunctions tend to transfer to zero conjunctions and "before," "after" and "since" introducing non-finite "-ing" clauses should be included into the grammatical category of conjunctive prepositions.