'at' and 'in'

Lim Chiu Lan from Malaysia asks about prepositional phrases:

Would you be good enough to explain to me what is the difference between these prepositional phrases: good at and good in?

Which of the following is correct: 1) 'I'm good at English' or 2) 'I'm good in English' and 1) 'I'm good at football' or 2) 'I'm good in football'? 

To be good at and to be good in are often interchangeable, Lim, 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.'