Is it wrong to say "expert of something", not "expert on something"? What's the difference?
"Expert of something" should not be used. You can say "expert on something" or "expert in something". You can use "expert on something" to refer to a specific subject one is an expert on. "Expert in something" can be used to describe a field of study one is an expert in. Ex: She's an expert in education. Ex: He's an expert on teaching English.