When is "su" used as in "What do you write ON" and when as in "They write about her" (Loro scrivono SU di lei). Has it something to do with the "di"?
Yeah, in "Loro scrivno su di lei" su and di means about, so by that explanation it could've translated to "About what are you writing?" And now, when I hovered on "su" it says on ...
it is right to say:
"about what do you write?"
I don't think so. The question is asking what (material) are you using to write on. Yours is asking what a person is writing about.
"on what do you write"