During a lesson on clause combinations, an English teacher explained that coordinators join two independent clauses of equal grammatical status, while subordinators establish a hierarchical relationship between a main clause and a dependent one. She wrote: "Although she was exhausted, she finished the project on time" and asked why "although" cannot be replaced by "but" without altering the grammatical relationship between the clauses. The teacher explained that "although" functions as a subordinator because it: