Why is Kawy genitive in this phrase?
I would expect 'Kubek kawę' as there is no "quantity of" coffee in an empty coffee cup?
Well, the fact that it's a cup is kinda a quantity itself, it's probably as much coffee as the cup can hold ;)
Anyway, you can assume that every usage of the English "of" triggers Genitive in Polish. Maybe there is some exception, but nothing comes to my mind now.
Emmm. "she buys a cap of coffee" - is wrong Am I not see some difference between mug and cup here?
'cup' works, so I guess it's because of the typo (you wrote 'cap' in your quote).
If I google 'cup', most of the graphics that I see is what I'd call "filiżanka", but true, some of them are "kubek".
Is that what the Polish would call one of these https://www.google.com/search?tbm=isch&source=hp&biw=1280&bih=800&q=cup+of+coffee&oq=Cup+o&gs_l=img.1.1.0l10.3116.4448.0.56184.108.40.206.0.0.0.206.830.0j3j2.5.0....0...1.1j4.64.img..2.5.826.0..35i39k1.0.Ubl85fJ4pUY#imgrc=wA0DfBLgsDhgqM: