Why does the electric field inside a conductor becomes zero when it is placed in an external electric field?

When a conductor is placed in an external electric field, free charges within the conductor move in response to that field. This movement continues until the internal distribution of charges creates an opposing electric field that exactly cancels the external field inside the conductor. As a result, the electric field within the conductor becomes zero in electrostatic equilibrium, ensuring that no net force acts on the charges inside. This is a fundamental property of conductors in electrostatics.