We learn about the dramatic changes in Minnie Foster, after she becomes Mrs. John Wright, through the commentary of Mrs. Hale.
As a fellow farmer's wife and country woman, Mrs. Hale has witnessed first hand how things go in the lives of her social peers. This said, she was quite aware that something was not right after Minnie married John.
Mrs. Hale says that, when Minnie was single, she was a girl who sang like a bird in the church choir, and that:
[s]he used to wear pretty clothes and be lively, when she was Minnie Foster, one of the town girls singing in the choir. But that—oh, that was thirty years ago.
Another indication of the deep changes about to take over Minnie Foster after marriage have to do with her overall appearance before and after.
I wish you'd seen Minnie Foster when she wore a white dress with blue ribbons and stood up there in the choir and sang.
After she became Mrs. Wright, Minnie Foster underwent a profound change. A close reading will indicate that she was a victim of domestic abuse by her husband. The abuse branched out into a number of consequences, including Minnie feeling ashamed and "shabby" about herself, to the point of not socializing anymore.
Arguably, John Wright may have controlled the way she looked, whom she spoke to, and every activity she did. As a result, a lonely farm life becomes even lonelier and more isolated. Minnie must have been quite miserable.
she kept . . . much to herself. She didn't even belong to the Ladies Aid. . . . she felt she couldn't do her part, and then you don't enjoy things when you feel shabby.
Therefore, Minnie Foster was a woman who seemed content with who she was and with the life she was leading during the years that she was single. She was cheerful, artistic, and expressed herself through music and singing.
After marrying John Wright, she was taken over entirely by her husband, and her quality of life diminished to the point of her being trapped in her own home with an abusive man. This is the same man who wrung the neck of Minnie's only companion, a canary. It is the general understanding of the play that the wringing of the bird's neck is the event that caused Minnie to snap and kill John Wright.