"The sick woman goes to the doctor."
Translation:Femina aegra ad medicum it.
don't take my word for it because in truth i'm at sea with all this too, but as I understand it: the two prepositions are ad, which is 'to' and ab, which is 'from'.
however 'ab' behaves like the english word 'an' and loses the b when the following word has a consonant.
So you get 'ab urbe', 'from the city', but 'a foro', from the market. the preposition is the same in both sentences.