If I'm playing over-the-board chess and my opponent makes a move that put himself into check what happens from there? Do I simply capture his king or would they be allowed to take their move back as it is illegal to put yourself into check?
Putting yourself in check is not a legal move. They are REQUIRED to take the move back.
Playing online, you couldn't even do it. The game interface won't accept illegal moves, so it would just reject that move and force him to play something else (ie: some LEGAL move).
Playing OTB (over-the board, in real life) the move is still illegal. He must retract the move and play something else instead. If the piece that he touched has any possible legal move, then he MUST play that. Otherwise, he must just make some legal move.
If clocks are being used, the player who made the illegal move is penalized by having two extra minutes ADDED to the opponent's clock (giving the opponent some extra thinking time) to make up for the distraction caused by making and then retracting an illegal move.
If I'm playing over-the-board chess and my opponent makes a move that put himself into check what happens from there? Do I simply capture his king or would they be allowed to take their move back as it is illegal to put yourself into check?