I was on the phone recently getting some tech support. As is often the case, I was talking with someone from India. I wanted to know whether I was doing something correctly and, after explaining it to the technician, I asked if I was doing it in the right way. “Of course!” he responded.
To me that sounded like “Obviously,” which is what it would mean if I used it in that context. My immediate response was, “Don’t talk to me like that. It’s not obvious to me!”
Later I thought about it and realized a few things about the expression. Commonly, there are two meanings for “of course.”
One meaning is: “It’s obvious.” This could also be interpreted as: “You are an idiot for not realizing that.” If someone asks you how to do something and you are irritated with them, say “of course” if you want them to feel like it is totally obvious and they are probably a little slow to not realize it. This isn’t so polite when someone is asking about themselves. But if someone asks something about you, for example if you are really going to do something — as in: “Are you really going to try sky diving?” – you can say “of course” to mean they shouldn’t doubt it. This sounds polite.
The other meaning is the one my tech support person really meant. If I am visiting you and want to take something from the fridge — for example: “Can I get some milk from the fridge for my coffee?” – if you answer “of course,” it would be interpreted as “please, you are welcome to take what you want.” In this case, it has a similar meaning to the first but it really means “of course you can take what you want.” It sounds generous and friendly.
The main thing to watch out for is when someone is unsure about their own knowledge and/or ability. Then you should probably just say “Yes.”