Question: When can a player be substituted in soccer?
Disambiguated questions and answers: question: When exactly in the course of a game can a player be substituted in soccer?
In most matches, when can a player be substituted in soccer in terms of frequency?
answer: during a stoppage in play and with the permission of the referee
three substitutions during a game and a fourth substitute during extra time
Natural Questions answer: during a stoppage in play (from the Wikipedia article "Substitute (association football)")