"Mom stirs her coffee with a spoon."
Translation:엄마께서는 숟가락으로 커피를 저으세요.
ㅅ is sometimes discarded during conjugation. For example: 젓다->저으세요. 젓다->저으세요. 낫다->나으세요. But sometimes ㅅ is not discarded. For example: 씻다->씻으세요. 웃다->웃으세요. The rule for the difference is what only linguists know because the reason relates to the archaic forms of those words... There's no way but just memorize the exceptions.