What is an impossible checkmate? If you open any book for beginners it will inform you that a king and a lonely knight cannot checkmate an opponent's king. Moreover, if you only have a king and a knight and your opponent lost on time in an Internet blitz game, you'll get a message on many playing servers that your opponent has lost on time but it is still a draw due to the insufficient mating material. Of course you can checkmate your opponent with a lonely knight, but your opponent needs to have some pawns or pieces. Here is one of the oldest chess positions that proves the point:
This is probably the first known example of so-called smothered checkmate. This pattern is well known and relatively popular in practical games, but I have never seen a position from a real game where the smothered checkmate would be delivered by the last knight where the winner has no other pawns or pieces (except a king) left on the board.
Now let's take a look at another 'golden' oldie:
It is interesting to note that this position is winning for White even if it is Black to move:
If you say that this kind of a position can never happen in a real tournament game, then I would respectfully disagree. Here is the game played in one of our local tournaments. I doubt that any of the opponents is going to be particularly proud of this game, so I will withdraw their names.
In conclusion, let me offer you a really tough puzzle where White at the end checkmates the black king with his lonely knight.
To be continued...