Frankly, leuleu, I have never seen it presented so simply, and I doubt that the explanation given you is complete or correct. I am unable to see any clear rationale for the difference between the use of ‘to’ and ‘for’ myself, and so did not respond to your first post.
Here is another explanation (directed to another poster) by a friend of mine, which may be useful:
[i]No. There isn’t really a rule, although generally the preposition that introduces the interested person usually depends on the adjective. There is usually a more-used preposition and perhaps another that is also acceptable.
On the other hand, you may wish to put this into a broader context. The full form of statements in the pattern ‘useful to’ (and others like it) may be thought of as
‘It is [useful / essential / helpful / … ] TO/FOR the interested person FOR subject TO predicate’–
– where ‘the interested person’ is the person to whom the situation is [useful / essential / helpful / …] and ‘FOR subject TO predicate’ expresses the situation which is [useful / essential / helpful / …]
When the interested person is normally introduced by the preposition FOR, there is potential for confusion between the interested person and the subject of the situation.
‘For his father to talk about the old country is exciting for Frank.’
(FOR subject TO predicate) is exciting (FOR Frank), where Frank is the interested person.
‘It is exciting for Frank for his father to talk about the old country.’
‘It is exciting (FOR Frank) (FOR subject TO predicate).’
So when you say ‘It is useful to me’, you are mentioning the interested person, but not the subject of the situation.
– ‘I can’t see how writing down all those reminders can be useful.’’
– ‘It is useful to me.’ (The situation is ‘FOR me TO write down all those reminders’, and it’s omitted.)
When you say ‘It is useful for me’, you are theoretically mentioning the subject of the situation, but not the interested person. However, when the predicate of the situation is not mentioned, the person introduced by FOR is taken to be the interested person anyway!
So in ‘It is useful to him for me to answer the phones while he is away from the office’, you cannot say, meaning the same thing in abbreviated form, ‘It is useful for me’, but you can say ‘It is useful for him’ or ‘It is useful to him’.
Consistent with this is the fact that in English we cannot have: *‘For whom is it useful to him to answer the phone while he is away from the office?’
This is a fairly advanced topic in English grammar. For more information, consult Syntactic Argumentation and the Structure of English by Soames and Perlmutter. (Section 63)
In the meantime, you may simply want to remember that ‘useful to’ is more usual and ‘useful for’ is also acceptable.
In addition, I (MM) could probably find some examples using ‘with’; I doubt it is non-existent-- at least for the ’ With me, of course, it is different’ type.