he ran to the station as ( ) as he could to catch the train

你好,
这边如果要用quick,应该填写quickly
因为动词后面要用副词~
as...as的结构中间的词也要受前面动词的影响~
fast本身可以作为副词使用~
祝：学习进步,更上一层楼! (*^__^*)
不明白的再问哟,请及时采纳,多谢!~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~