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.