There are a number of ways to interpret this question, as King Lear is a more thematically complex play than, say, Othello or Macbeth. However, I would say the main conflict is between Lear's false vision of reality and physical reality itself. Many themes then spin out of that main conflict between illusion and reality, a conflict that is laid out for us in the first act.

In this act, Lear is ready to retire from active monarchial duty and turn his kingdom over to his three daughters, as long as they pledge love and fealty to him. The first two do this readily, with flattering but empty words; the third refuses to flatter. Lear, too long removed from reality, believes his two duplicitous eldest daughters' words and rejects Cordelia in fury. His Fool tells Lear that he is the biggest fool of all in dividing his kingdom and giving up his power—and in exiling Cordelia.

The Fool proves to be wiser than the king, who has to find out the hard way that his power does not reside in people's innate love of his person, but in the actual manifestations of his power—land, wealth, and armies—that he has given away. Lear's daughters' dizzying speed in casting him aside, even cruelly throwing him out to fend for himself on the heath in a storm, reeducates Lear in what reality is.

Lear learns to regret trusting his eldest daughters and rejecting the loyal one, and as he suffers on the heath, without food or shelter, he learns to regret that he did not do more for the poor of his kingdom while he had the chance.

Lear ends up a far wiser and sobered man after he is stripped of his power, but even at the end of the play, it is difficult for him to accept reality: although Cordelia is obviously dead, he still wants to test this fact by putting a mirror to her mouth to see if it will pick up the mist of breath.

Through Lear's conflict with concrete, physical reality, audiences are reminded to trust in deeds and materiality rather than words and flattery.

What are the main themes in King Lear by William Shakespeare?

One of the main themes in King Lear is the relationship between appearance and reality. When Lear calls upon his daughters to profess their love for him publicly, he's much more concerned with the appearance of their love rather than the substance, the reality. Cordelia is the only one of his daughters who truly loves him, yet she is also the only one who refuses to play along with the whole charade. Lear mistakes Cordelia's reticence for lack of feeling and immediately banishes her. By the same token, Regan and Goneril are rewarded despite the fact that their hearts are full of loathing and contempt for their foolish father.

Even after Lear has divided up his kingdom, he still expects to be treated as a king. That he isn't is a major source of his subsequent descent into madness. Once again, Lear is fixated on appearances, completely ignoring the reality that he no longer enjoys any power. Gloucester's relations with his own children provide a notable parallel with Lear's. He also realizes much too late which one of his children really loves him. For much of the play, Gloucester is quick to believe the lies and slander spread by Edmond against Edgar.

In such a toxic, treacherous environment, where the boundaries between appearance and reality are so frequently blurred, it's no surprise that various characters are required to put on disguises in order to act nobly. The unfailingly loyal Kent pretends to be Caius to be able to serve his master; Edgar disguises himself as Poor Tom, a crazed, half-naked beggar, in order to move about freely and deal with the slanderous accusations made against him.

What are the main themes in King Lear by William Shakespeare?

King Lear by William Shakespeare is focused on themes of love, family, and aging. King Lear expresses one of the major themes, that of the duty of love between parent and child and its betrayal by the child's ingratitude when he states:

　"Ingratitude, thou marble-hearted fiend,
More hideous, when thou show'st thee in a child,
Than the sea-monster." -

 King Lear, 1.4.283

One major thematic concern is how one distinguishes the true love of Cordelia for her father from the false love and flattery displayed by the other two sisters. The adultery sub-plot echoes the sister's ingratitude as it also emphasizes improper relationships and inconstancy. The issue of children's duty to parents leads to the marriage theme in another way in asking the proper degree for affections to shift from the birth family to the new family, and how conflicting obligations and priorities are handled

What is the main theme, or the main idea, in Shakespeare's play King Lear?

My perspective on Shakespeare'sKing Learis that the main theme is the painful fact that each succeeding generation pushes out the generation that came before it and was responsible for its creation. In order to evaluate the theme or meaning of the play, one would have to consider both the main plot involving Lear and his daughters and the subplot involving Gloucester and his two sons. It is natural for a father to love his children, but it is not necessarily natural for children to love their father. Each new generation has to fight to find and maintain a niche on the small, rotating globe and then reproduce, and this means displacing those who are trying to hold on to their little niches.

In John Keats' "Ode to a Nightingale" he uses the phrase "No hungry generations tread thee down." That is the truth about life: the hungry generations of all the various species, including homo sapiens, are forced to "tread down" those that came before them. When Lear and Gloucester get together in the open fields, after both have been displaced from their homes and all their property, they commiserate about the cruelty of life. It is a dog-eat-dog struggle for existence and reproduction. Shakespeare had a hard life. He knew what it was like to struggle for existence.

There are some good characters in King Lear. Edgar and Cordelia are the notable examples. Shakespeare may be using them mainly as "foils" to offset the wicked Goneril, Regan, and Edmund. There would have to be some good people in the world; otherwise we wouldn't recognize the wickedness, greed, and selfishness of the others. But the thrust of life is always for one generation to displace the generation that created it.

It is interesting that so many of Shakespeare's plays have themes related to the idea of usurpation. These include Hamlet, Macbeth, Richard II, Richard III, Henry IV, The Tempest,and AsYou Like It. In Julius Caesar, Brutus and Cassius seize power from Caesar and then Antony and Octavius seize power from them. In Othello, Iago usurps Cassio's position and may be hoping to displace Othello. King Learis about usurpation, in a sense, because Lear's daughters get his kingdom under false pretenses, and Edmund steals Gloucester's title and lands from both his father and his brother.

What is the main theme, or the main idea, in Shakespeare's play King Lear?

What is the main theme, or the main idea, in Shakespeare's play King Lear?

I agree with each assessment above and I'd add one more possible argument for a central theme. King Lear can be read as a play about identity.

Lear is doomed when he attempts to change who he is. It is not the crimes and ambitions of his past that lead him to failure and destruction as much as it is his attempt to do the impossible, to be what he is not.  

What is the main theme, or the main idea, in Shakespeare's play King Lear?

Another interesting perspective came from a lawyer who once told me the King Lear is a good example of what not to do during retirement. He said that King Lear was crazy to give out his children's inheritance before he died because that is exactly what he has seen children do to parents in lawsuits. Children who are anxious to get "what they deserve" from their parents merely use them in the end.  I thought that was certainly an interesting perspective of the parent/child theme.

What is the main theme, or the main idea, in Shakespeare's play King Lear?

I can't argue with betrayal, but I also think that a major theme is the ultimate humanity of us all. Lear is full of himself, claiming absolute paternal control over his daughters, which is why he gets so angry at Cordelia. This absolute control erodes very quickly along with any vestige of Lear's majesty as the consequences of his decision to alienate his daughter and the fact that he is surrounded by wicked schemers literally causes the world he has constructed for himself to come crashing down around him. It is not going out on a limb to say this, but I have come to regard King Lear as Shakespeare's finest play.

What is the main theme, or the main idea, in Shakespeare's play King Lear?

I think that the main theme is betrayal.  The play shows how people (like Lear) can put their trust in the wrong people.  It then goes on to show what the impact of betrayal is on the person who has been betrayed.

What is the main theme in King Lear?

The central theme of this play is related to self-knowledge (and the dangers inherent in a lack of self-knowledge). This theme is closely linked to honesty, illusion, delusion, and deceit. 

King Lear initiates the action of the play with a fundamentally false projection of his own character, attempting to abruptly change his social and familial position by stepping down from the throne.

Lear earns his tragedy through a disruption of both the political and the natural order of things.

However, Lear does not wish to change himself. He wishes to still be seen as a king and a father, though he has essentially abdicated these positions in giving away his title. 

Lear has a mistaken view of his own identity, believing that he can maintain his clout, his self-respect, and his moral impunity after taking off the crown. He is wrong. The conflicts of the story are, in large part, conflicts borne directly out of this mistake. 

Lear loses everything - his family, his prestige, and his moral impunity. He is made to face the evils he had done as king. He is made to realize that the trappings of the crown were major components of his identity. He recognizes, in the end, that a person cannot simply choose to upset the social order.

Roles are not so flexible nor as arbitrary as he first believed. 

What transforms Lear after his experience in raw nature is his acceptance of his own natural limitations, of his place within, and not above, the natural world