Question: When can a player be substituted in soccer?
Answer: during a stoppage in play
Disambiguated annotations:
qaPairs: question: When exactly in the course of a game can a player be substituted in soccer?
answer: during a stoppage in play and with the permission of the referee
question: In most matches, when can a player be substituted in soccer in terms of frequency?
answer: three substitutions during a game and a fourth substitute during extra time