Keats's Isabella closely follows the plot of Boccaccio’s "Lisabetta" from the Decameron. In both stories, Lisabetta/Isabella falls in love with Lorenzo. Her brothers disapprove and kill Lorenzo, telling her he has gone off on a voyage. She pines for him until he comes to her in a dream, tells her has been killed, and indicates where his body is. Lisabetta/Isabella goes with a companion, finds the body and digs it up. She can not take the entire corpse back with her, so she cuts off the head and plants it in a large basil pot. When her brothers begin to wonder why she is crying over the pot all the time, they take it and discover Lorenzo's head. This frightens them and they flee. Lisabetta/Isabella, pining for her basil pot, dies of grief.
One difference between the two versions is context. "Lisabetta" is part of a series of stories that have unhappy endings. As Filomena, the "Lisabetta" storyteller states, her story is not about royal people, as the previous story by Elissa was. Filomena explains:
My story, dear ladies, is not about such wealthy people as those in the story that Elissa has just told, but as it happens, it is no less pitiful than that ...
This framing brings the idea of social class to the forefront.
Keats's standalone poem lacks this framing. It is not part of a series about tragic love. It is not a story told in contrast to a tale about royalty. In it, social class is what keeps the lovers apart, but in Keats's case, the brothers are explicitly framed as ambitious more than snobbish: they want Isabella to make a wealthy and important match. In Boccaccio's case, the brothers are far more concerned with the "shame" of their sister having an affair with a man not of a class they feel she can marry into than who she might marry in the future.
Boccaccio's story is more concise, moving along quickly. Keats lingers, extending the time it takes the lovers to get over their shyness and confess their love for one another, devoting nine stanzas to their hesitations and pinings before they consummate their relationship. This allows the reader to get a fuller understanding of the delicacy of the lovers and foreshadows, too, Isabella's pining after Lorenzo's death.
In Boccaccio, the two protagonists are lovers within a paragraph. This is more in keeping with the crude, lustful versions of love that emerge from the Decameron. Here we learn:
Elisabetta saw him several times and though it may seem strange she fell in love with him.
The lower-class Lorenzo here responds more opportunistically when he understands Elizabetta loves him:
Once Lorenzo became aware of this, after seeing her a few times he too fell in love with her ...
In Keats, a potentially sordid love affair is made ethereal and exalted in way it is not in the original version. In Keats's version, too, there are only two brothers, whereas in Boccaccio there are at least three. Bocaccio also puts us directly into the thoughts the eldest brother, while Keats's narrative views the brothers from afar. In his more romanticized version, the brothers take Lorenzo on a hunt to kill him, whereas the more pragmatic earlier version frames this as a business trip. In Keats's version, perhaps alluding to Romeo and Juliet, Isabella is accompanied to her lover's grave by her nurse.
Overall, Keats fleshes out Bocaccio's more pragmatic, concise tale to heighten the intensity and the spirituality of the love affair between Isabella and Lorenzo.