What is the difference between a main clause and a subordinate clause?

Robert was not waiting for her in the little parlor. he was nowhere at hand. the house was empty. but he had scrawled on a piece of paper that lay in the lamplight: "i love you. good-by—because i love you." what significant idea, presented throughout the novel, does robert's final departure and note highlight? edna is ultimately alone in her rebirth. edna needs to find solace in her children. edna will never be truly loved by anyone. edna should not have forsaken her family.