请教关于first的用法

takutokinki

Although early soap operas were first aired on evening radio in the 1920's, they had moved to the daytime hours of the 1930's when the evening schedule became crowded with comedians and variety shows.
请从语法角度解释为什么first一定要在aired之前？

jessiecg

不知道啊。。
顶一个。。