"My mom has black coffee."
Translation:Mae gan fy mam goffi du.
gan is not a verb - it's a preposition meaning "with".
A more literal translation is "There is black coffee with my mother", or even more literally, "Is with my mother coffee black".
So you can't split up the gan from the fy mam.
I believe that some use gyda for "with" instead of gan and that then the phrase would come later: Mae coffi du gyda fy mam "Is coffee black with my mother = There is black coffee with my mother = My mother has black coffee".