Preposition At/in/on: They're all expert at/in/on this field

[Tung Quoc]

Dear teachers,

Please read:

1. They're all expert at/in/on this field.
2. He's expert at/in/on making cheap.

Do you choose "at", "in" or "on" for each sentence? For each sentence, all are possible and interchangeable? If not, how to distinguish when using "at", when using "in" , when using "on"?

Quoc

[Mister Micawber]

.
One short, simplistic answer:

At a skill
In a field
On a topic.
.
_________________
Native English teacher at Mister Micawber's

[Tung Quoc]

Thanks Sir,

But what does "one short" mean? It's an expression?

Quoc

[Torsten]

Hi Quoc,

Mr. Micawber gave you a short and simplistic answer.

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