是'i am good at swim'还是'i am good at swimming'?

no,no,no！不要被误导了，我们可以说 "go for a swim(名词）" OR "have a swim（名词）"
但是at是一个介词，后面应该跟一个动词ing形式，所以是"i am good at swimming."
当然介词后面也可以加名词，例如"i am good at study."

是'i am good at swiming' swim 本身可以做动词用的 加了
ing 可以做名词用 和read study一样

swim（n.）和swimming分别是swim(vi.)的同根名次和动名词，都是名词可以做宾语嘛，所以都是对的。

是i am good at swimming'

I am good at swimming.
At是介词,后要跟动名词.

be good at 中 at是介词 后面接动词的 ing形式·