"우리들의 엄마가 떠났어요."
Translation:Our mom left.
It can be transformed to a honorific even: 돌아가시다 (합쇼체 past tense: 회장님이 돌아가셨습니다) to indicate that someone passed away. You would usually hear doctors say this when they declare the patient's death or inform the families, or from anyone as long as they honor the dead. Alternatively, there is also 사망하셨습니다.