“every + 基数词 + 复数名词” 和 “every + 序数词 + 单数名词” 的意思一样。前者的含义是 “每数几个为一组”,后者的含义为 “每数到第几个为一组”,其实都表示 “每隔几个”。
如:
every four years / every fourth years 每隔四年
every two hours / every second hour 每隔两个小时
every three seats / every third seat 每隔三个座位;
every ten students / every tenth student 每隔十个学生