every+ 基数词+ 复数名词=every+ 序数词+ 单数名词
作“每(多少)”解
如： every three days 每三天或每隔两天
相当于 every third day
"every other+单数名词”
意思是“每隔一.”
如：every other day 每隔一天 every other tree 每隔一棵树
every few+复数名词”
意思是“每隔几.”
如：every few days（每隔几天）