What is the relation between the main plot and the sub plot in King Lear?
The sub-plot mirrors the main plot. Lear, excepting Cordelia, has unfaithful daughters; Gloucester, excepting Edgar, has an unfaithful son. Both plots describe the machinations the unfaithful children of the king and duke, Goneril and Regan disposessing Lear and plotting against each other, Edumund supplanting and smearing Edgar to inherit fully his father Gloucester's estate. As the two brothers, Edgar and Edmund are at odds, so are the two sisters, Goneril and Regan after Edmund starts a love triangle and causes them to do each other in over their jealousy for him. An interesting parallel occurs after Lear is outcast and becomes more deranged. He, along with his faithful servant Kent, meet Edgar, who is playing the madman as a disguise. Here we have two characters, both disposessed of what was rightfully theirs; Lear by his unfaithful daughters and Edgar by his brother. There's an interesting contrast in the actions of the siblings -- The evil sisters do themselves in; in the case of the Gloucester brothers, one kills the other, Edgar, the "good" son surviving. Unfortunately, the "good" daughter, Cordelia, innocent to all the evil caused by her sisters, is killed as well. Edmund, therefore, wipes out Lear's progeny.