what is the right thing to say:
it is useful to me or for me.
but what is the difference?
I prefer "useful to me." Generally, "to" signifies intended to be used in a specific way, whereas "for" signifies belonging, suitability or ownership.
I agree that, while they generally mean roughly the same thing, nuances of difference could be extracted. But I can't think of anything that is useful to me that isn't useful for me or vice versa. Can you?
It's strange that all my dictionaries, including Oxford, Longman, Cambridge and MacMillan, just say 'useful to sb', and 'useful for sth/ for doing sth'.
Sorry to cause confusion. I am likely not expressing my ideas well.
I mean that, given the subtle differences in meaning between "to" and "for" as I see them, the concept of usefulness to a person seems to match the "to" meaning (intended use) better than the "for" meaning (a natural fit).
To my ears, "useful to me" sounds more natural than "useful for me" to boot.