Final answer:
Mercury will go through three periods of retrograde in 2021.
Step-by-step explanation:
Mercury goes through periods of retrograde when its apparent motion in the sky temporarily reverses. This is most evident with Mercury and Venus, which shuttle back and forth across the position of the Sun. In 2021, Mercury will go through three periods of retrograde. During these periods, it will appear to move backwards in the sky before resuming its normal forward motion.