They still remember the happy years---in the country

They still remember the happy years是主句,已经完整。___in the country做years的定语,可以有两种表达：1,分词短语做定语。因为years与spend是被动关系,所以用spent。2。定语从句。因为从句中不缺少时间状语,所以不用when做引导词,若用定语从句,则为：which they spent together in the country。在这里,从句中的主语不能省。