Never once has the old couple quarrelled with each other sin

这里的主语是the old couple 用have 也是正确的哦·1） couple表示“夫妇”“夫妻”且用作主语时,谓语动词可用单数（视为整体,尤其是当有each,every,only one等修饰时）或复数（考虑其个体）。如：The couple were married last week。这一对夫妇是上星期结的婚。Each couple was asked to complete a form。要求每对夫妇填一张表。2）有时语境上不明确强调个体还是整体,则用单数或复数均可：The couple is [are] spending its [their] honeymoon。这对新婚夫妻在度蜜月。【俊狼猎英】 欢迎追问