If you are new to chess you need to understand one of the oldest tricks in chess, the four move checkmate. It is quite simple to make quick work of less experienced player by this method. Due to its fast speed, the inexperienced player will never see it coming. Therefore it is important to be able to detect the proper board positions of the pieces in order to properly defend one's king.
Characteristic of the four-move checkmate is the board positions of the Queen and Bishop. If you see your opponent moving their Queen and Bishop early in the game, it's a big sign that they are setting up for the four-move checkmate. Also, do not be fooled into thinking that this checkmate should be done in four moves either. Oftentimes, even players who know about the four-move checkmate are defeated by it because their opponent was able to disguise their intentions with distracting moves. You should always be on the watch when your opponent's Queen is attacking the King's Bishop Pawn.
If you are white, moving the king pawn forward two squares followed by black moving their king pawn forward two squares start the process. Then white moves king bishop to attack blacks king bishop pawn. If black does not see this coming white places its queen in a position to attack blacks king bishop pawn. If black does not defend the coming attack then white has checkmate the next move by taking blacks king bishop pawn with the queen.
To defend against this attack black could have simply moved the knight in front of the king bishop pawn. Another defense is to simply move the king bishop pawn forward one square. Either of these methods is acceptable.