ブロ友さんが次の表現は間違ってるんじゃあないか、皆さんの意見は？
というのがありました。ちょっと面白い内容だったので、少し内容を変えて出題してみます。（？）
問題 さて、次の英語の文章で正しいと思う文の番号を選んで下さい。
1) I'm good at English.
2) I'm good in English.
3) I'm good at football.
4) I'm good in football.
これに対する答え
Ⅰ
To be good at and to be good in are often interchangeable, and there is no easy rule to follow. In simple statements, like the ones you have quoted, the standard form appears to be good at as in 'I'm not very good at football'.
However, in this following sentence, to be good in seems more likely than to be good at, i.e:
'He was the best in the class in French, but in mathematics and chemistry he was not so good.'
This is perhaps because with other expressions or verbs denoting assessment or ranking, the preposition in would be required, thus:
'In pharmacology she obtained/scored/gained/attained the highest marks.'
Ⅱ
1) I'm good at English.
2) I'm good at football.
Note that "I'm good in English." is a valid sentence. It could refer to an English class.
1) I'm good at English.
2) I'm good at football.
Note that "I'm good in English." is a valid sentence. It could refer to an English class.
But the answer you are most likely looking for (the more common one) is 'at'.
★さあ、どうだったでしょう？奥が深いですね～。