Final answer:
A private party can arrest someone if they witness a crime being committed or have reasonable grounds to believe that a crime has been committed.
Step-by-step explanation:
In certain circumstances, a private party may arrest another individual. This is known as a citizen's arrest. A private party can arrest someone if they witness a crime being committed or have reasonable grounds to believe that a crime has been committed. Examples of situations where a private party may make an arrest include catching someone in the act of stealing or witnessing a physical assault.