Reflections on the Grammatical Category of before, after and since Introducing Non-finite -ing Clauses: A Corpus Approach

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.