These are some phrases that advance the central theme of the story, that of the oppressive position women were forced to accept in nineteenth-century marriages.
"She was young, with a fair, calm face, whose lines bespoke repression..."
The author does not spend a lot of time describing Mrs. Mallard's physical appearance. It is significant that "repression" is one of the few words she chooses to describe her.
"She was beginning to recognize this thing that was approaching to possess her, and she was striving to beat it back with her will..."
The realization of her situation and her longing for freedom are taboo during her time, and Mrs. Mallard does her utmost to prevent herself from acknowledging these inclinations in herself, knowing they can lead only to destruction.
"She would live for herself. There would be no powerful will bending hers..."
Women are subjugated by the will of their husbands. What they want is not important; it is what the man wants that takes precedence during these times. With the removal of her husband, Mrs. Mallard will be able to experience a freedom which has long been denied her.
"What could love, the unsolved mystery, count for in face of this possession of self-assertion which she suddenly recognized as the strongest impulse of her being!"
Self-realization is more important even than love by Mrs. Mallard; it is all-consuming.
"She breathed a quick prayer that life might be long. It was only yesterday she had thought with a shudder that life might be long."
Mrs. Mallard's married life was so odious to her that she did not feel it was worth living. This was true even though her husband was a kind man; the impulse towards self-realization inside herself was that strong.
"...she had died of heart disease - of joy that kills."
This statement has multiple meanings. "Heart disease" refers to Mrs. Mallard's physical ailment as well as to her longing. The "joy that kills" is the sense of identity and freedom she craves; in the society and times she lives in, it is a forbidden thing that can only result in ruin.