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I was recently engaged in a discussion with a Bulgarian english teacher on Skype. I was telling him that I was good IN pronunciation and he immediately corrected me with "good AT".
I was puzzled by this, so I googled about it.
So I found this BBC english place
http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv8.shtml

It was written that there's almost no difference between these 2. But the bulgarian friend insisted that he was right with "good AT".
Althought "Good at sex" is right, but "Good at chemistry/pronunciation/medicine/bed" sounds not right to me.

So anyone? Enlighten me please!!!
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I would say that "in" is somewhat more appropriate for a subject or field of study, and "at" is somewhat more appropriate for a skill or activity.
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khoffI would say that "in" is somewhat more appropriate for a subject or field of study, and "at" is somewhat more appropriate for a skill or activity.

I think this comes close to my differentiation: good in [noun, such as math, electronics]; good at [ ___-ing, such as translating, cooking].
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Comments  
Hi,

They both sound OK to me, and often no real difference is intended by the speaker.

Here are a few comments on nuances that I perceive.

good at pronunciation More formally correct. Can be taken to mean you do it well, ie you pronounce words well. Most native speakers are good at pronunciation.

good in pronunciation A bit more informal, by which I mean that I'd consider saying it in another way in a formal context. Sounds a bit like you have knowledge about pronunciation. Perhaps you can teach it.

Best wishes, Clive
"Good at pronunciation" is the only version that my ear accepts without cringing.

Your other examples:

"Good at sex"
"Good at chemistry" "Good in chemistry"
"Good at medicine" "Good in medicine"
"Good in bed"

These all seem okay to me but the ones in blue are a bit rough on the ear.
"good in pronunciation" sounds quite wrong to me. In fact, without any explanatory context I might not even understand that it was supposed to mean "good at pronunciation". Perhaps there are some regional differences here (I'm British English).

As far as my usage is concerned, I also don't agree with the emphasis of the statement on the web page you cite that "To be good at and to be good in are often interchangeable". In my usage, they are most often not interchangeable. "good at" is my preferred form in most cases; "good in" is, to me, either inferior or used only in certain special situations (such as, perhaps, the example sentence they give).

"good in bed" is fine but is somewhat different from your other examples because "in" is used to literally specify a location.

Google stats suggest that "good at" is overwhelmingly more common:

about 8,360 for "I'm good in" -"i'm good in bed"
about 1,090,000 for "I'm good at".
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To me, good at bed doesn't show ability in anything. Therefore, good in should be used instead.

To be good at sex expresses your ability in sex.
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PhilipI think this comes close to my differentiation: good in [noun, such as math, electronics]; good at [ ___-ing, such as translating, cooking
I don't think it is wrong to say "I am good at math." To me, 'good in' either doesn't fit in the sentence, or is not the only choice.