The bus which goes to the airport runs every half hour 哪句是主句 哪句是从句啊

The bus (which goes to the airport){定语从句修饰先行词the bus } runs every half hour .
The bus runs every half hour .是主句为什么我说 The bus which runs every half hour goes to the airport 说我错呢你这句语法上没错。不过意思变成“第半小时一趟的公共汽车去机场‘第一句的意思是’去机场的公共汽车每半小时一趟‘