A student asks:

I have had this question for over a year. 

How do you translate this: I am eating bread for dinner. Is it: Edo panem cenae? “For” is a hard word for me to translate; in this context should ‘dinner’ be in the dative?

Here is my reply:

Your question threw me.  I wasn’t really sure how to answer.  I had never thought of this myself.  Sometimes, when I am stumped in Latin, I will actually go take a look at the languages derived from Latin.
In order to answer your question, that is what I did.  Take a look.
I am eating bread for dinner.
Italian: sto mangiando il pane per cena
French: Je mange du pain pour le dîner
Spanish: estoy comiendo pan para la cena
Romanian: Mănânc pâine la cină
Esperanto: Mi manĝas panon por vespermanĝo
Portuguese: estou comendo pão para o jantar
The preposition per is the precursor to the highlighted, underlined words above.  Based on that, I would surmise the Romans would say something like:
Panem per cenam edo/manduco.  Turns out, you wouldn’t use pro at all.  Go with per.