I read “An assistant to a powerful man” and I was wondering if that’s the correct way to say it because I thought it would be better “an assistant of a powerful man”. Am I wrong? When should be used “to” and not “of”?
To identifies an intent: target/destination or direction of facing, moving, or progress.
Of identifies a connection: relationship, containing, ownership, reason or source.
There are of course exceptions, idioms, fixed phrases, and edge cases where the above doesn't apply.
It's possible for both of these to apply at once depending on what you
An assistant to a powerful man
The focus here is on your job or what you are doing.
An assistant of a powerful man
The focus here is on your relationship between yourself and the powerful man.
When I served as private secretary to a very high UK government official, the pronoun was 'to', as I have indicated. The word 'of' was never used in that context.
But when speaking of me to other people he would have said "Jeremy is my private secretary". Note the possessive. And I would certainly have been described as "head of Sir A B's office" - not "to".
I was "subordinate to" my boss but one "of Sir A B's subordinates".
When it comes to job titles it is not possible to state definite rules because different organisations will have their own individual language. In many decades of encounters with people who have assistants, I have never heard "assistant of' that does not mean that someone somewhere might use such a title, but in ordinary use "assistant to" is what people say.
There is no general rule for "when to use to and when to use of".
It is an unpredictable property of the particular word involved.
Assistant takes to for the principal person or role, not of: there is no way to predict this, you just have to learn it as part of the dictionary definition of assistant.