In this paper, I explore the complex domain of counterfactuality, arguing for its recognition as a discrete grammatical category. My analysis is based on the premise that counterfactuality, as a linguistic construct, encompasses propositions and scenarios that diverge from the empirical realities of our world. I then examine the use of counterfactual expressions in both subordinate clauses and main clauses, with a particular focus on their prevalence in subordinate clauses. These expressions are carefully categorized into three distinct types according to their semantic nature: conditional construction, concessive construction and contrastive construction. Finally, the functions of counterfactual expressions are delineated, revealing their multifaceted roles in serving emotional, preparatory, and inferential purposes within communication.