When you arrive at school,the first thing is to greet the teacher为什么不是greeting

题目

When you arrive at school,the first thing is to greet the teacher为什么不是greeting

答案

楼主是问为什么用to greet不用greeting?这是因为句中有first（其他序数词,以及the last等都可以）.比如He is always the first to come,and the last to leave.固定用法来的,其后的动词做定语或表语都有这个规律.