The long-distance relationship was difficult because they were apart for most of the year.

apart 是副词，由前缀 a-（表示"分离、远离"的状态）和 part（部分）组成，字面意思是"分成不同部分"，引申为"分开、相隔"的意思。在句中表示"身处不同地方"的状态。