They cannot take part in the game.
The game carries on.
The player is shown a yellow card.
The substitute must go back and wait for the correct time to enter (no card is shown).
Both players are shown a yellow card.
The substitute may continue to play, no card is shown, and the matter is reported to the appropriate authorities.
This must be prevented by the referee.
As soon as this is noticed, it must be corrected and reported to the appropriate authorities after the game.
A direct free kick or penalty kick is awarded, and a red or yellow card is shown (see: Red cards and Yellow cards).