What are the similarities between the subplot and mainplot in the "King Lear"?
Both the subplot and the main plot are intended to illustrate Shakespeare's thesis that each generation blindly creates the generation which will take over all its possessions and leave it to die. Lear has daughters and Gloucester has sons. This is intended to show that the playwright is dealing with a universal truth and not with a specific instance. When Lear is living out in the open country scrabbling for whatever food he can find to eat, including a mouse, he rails on the folly of copulation, which seems so pleasant when it happens but leads to such sorry consequences.
Behold yond simpering dame,
Whose face between her forks presages snow;
That minces virtue, and does shake the head
To hear of pleasure's name;
The fitchew, nor the soiled horse, goes to 't
With a more riotous appetite.
Down from the waist they are Centaurs,
Though women all above:
But to the girdle do the gods inherit,
Beneath is all the fiends';
There's hell, there's darkness, there's the sulphurous pit,
Burning, scalding, stench, consumption; fie, fie, fie! pah,
Lear sees how he has brought about his own destruction through his sexual appetite. Gloucester's case is worse. Lear at least conceived his hateful daughters "between lawful sheets," but Gloucester conceived Edmund through adultery and even brags about it to Kent in the opening scene because it shows what a lusty fellow he is. Edmund ends up with his father's property and title, while Gloucester ends up homeless and blind. Both old men are cold, filthy, and starving. Lear is eating mice! Gloucester is so disillusioned and embittered that he only wants to die. All of this is only symbolic of how one generation creates the generation which will supplant it and show no love or gratitude. It doesn't make sense for people to create people who are going to "tread them down," as Keats expresses it in "Ode to a Nightingale." But every generation does it. We are manipulated by programming of which we are unconscious.
Shakespeare expresses a very similar idea in his play Measure for Measure. Duke Vincentio disguised as a friar is visiting Claudio in his cell and gives him perhaps the most pessimistic assessment of human life to be found anywhere in Shakespeare, including the following:
Friend hast thou none;
For thine own bowels, which do call thee sire,
The mere effusion of thy proper loins,
Do curse the gout, serpigo, and the rheum,
For ending thee no sooner. (3.1)
Shakespeare needed a subplot to keep the dramatic and visual action going. Lear is absent throughout much of the middle part of the play. He refuses to accept his daughters' terms and goes out into the open country where he wanders aimlessly about. The subplot in which Edmund betrays both his brother and his father takes over as the space in which exciting things are happening. Without the subplot there would be a sort of huge hole in the middle of the play. Lear won't submit to his daughters and they won't relent. That is a dead end. Lear living like an animal has reached the critical point that Gloucester has yet to come to. Then when they meet by chance in an open field they are both destitute and disillusioned old men getting ready to die.
Only the man who attains old age acquires a complete and consistent mental picture of life; for he views it in its entirety and its natural course, yet in particular he sees it not merely from the point of entry, as do others, but also from that of departure. In this way, he fully perceives especially its utter vanity, whereas others are still always involved in the erroneous idea that everything may come right in the end. --Schopenhauer