Why are we not allowed to capture the King? Why do we have to corner the King in a checkmate? Why can't you kill him if your opponent did not notice that his king was in check? Because like in real life, killing the opposing King on a battlefield is not very profitable. It is much more useful to capture him alive and ask for a ransom, thus preventing a new King and getting a huge sum of money.
Also, in the era when chess was invented, playing a game where you won by killing a royal could have been seen as tantamount to a declaration of high treason.
Or perhaps you're taking him and his nation hostage, as a puppet state.
The King can move one space in any direction, compared to the Pawn who can move one space forward. The King is just a Pawn who has more opportunities and is given more weight in the game's rules.
You can't castle through check because the King would be taken En Passant.
Can occur when analyzing a written record of an already-completed game. For example, if White won a game and looked it over later, White might see that if Black had played some other response at some point instead of the actually-played line, it could have been devastating and Black could have won the game instead due to White's mistake.
Or there was an illegal moved played that no-one noticed that completely changed the game.
