I would take a long walk before going to bed.为什么go要用ing形式?

I would take a long walk before going to bed.为什么go要用ing形式?
I would take a long walk before going to bed.为什么go要用ing形式?

I would take a long walk before going to bed.为什么go要用ing形式?
before是介词,介词后面跟动词的ing形式.