The document discusses subordinate conjunctions, which are connecting words or phrases that introduce dependent clauses and join them to main clauses. It explains that subordinate conjunctions have two jobs: providing a necessary transition between clauses and reducing the importance of one clause to indicate which idea is more important. It categorizes subordinate conjunctions into six semantic categories based on meaning: time, concession, comparison, cause, condition, and place. Examples are provided for each category. The document concludes with exercises for practicing identifying and using subordinate conjunctions to combine sentences.