めもぶろ～目指せペラペラ！～: ～が得意です

「私は英語が得意ではありません」という英文を書こうとして

I'm not good at English.
I'm not good in English.

という、2つの文章を見つけたので、good in と good at の違いを教えてもらいました。

Q.
Would you tell me the difference between 'good at' and 'good in' ?

A.
Hmm... A good question.

The only reason I found to use 'good in' would be to describing a location or idea rather than an action.
Example:
There is no good in the world.
I found nothing good in breaking windows.

You'll usually use 'good at' when describing your( or someone else's ) proficiency doing something.

Example:
He is good at playing baseball.

Hope this helps!