Do you have to tell someone that they set them selves up for a check mate? For example if the queen is guarding the king on both ends and one person moves there queen to the left. Does the opposite person have to tell the opponent that they set them selves up?
If your question is "Do I have to tell my opponent that he just made a move from where checkmate is forced?", the answer is "No", you don't have to tell your opponent he is setting himself up for checkmate. In fact, that is rather the point of the game!
The only times you have to tell anything to your opponent is:
- When offering a draw
- When you want to adjust a piece.
If your question is "Do I have to tell my opponent that he is in checkmate?", the answer is still technically "No". Even the customary "Check" and "Checkmate" are optional.
Practically, if your opponent does not grasp that he is in checkmate it may be useful to point that out to him one way or another, and the traditional "Checkmate!" should do the trick.
I think you are confusing check and checkmate. Check is the situation where a king is under attack from another piece. Checkmate is where that is the case, the king's side has the move, but there is no move possible that stops the check.
Your opponent isn't allowed to play any move that puts himself into check. So if he does, you have to tell him, and he has to take the move back and play something else. With the same piece if possible (touch move rule).
In tournament play, there are often penalties involved if you play an illegal move, like some minutes extra time for the opponent, or the loss of the game (common in blitz, or if it's the second or third illegal move you play in the same game).