"Pyry, do you have a girlfriend?"
Translation:Pyry, onko sinulla tyttöystävää?
This is actually pretty difficult to explain in English but I will try my best!
- Onko sinulla tyttöystävää?
- Onko sinulla tyttöystävä?
- = Do you have a girlfriend?
- Onko sinulla uutta tyttöystävää?
- Onko sinulla uusi tyttöystävä?
- = Do you have a new girlfriend?
So as you see, four sentences in Finnish, only two in English. I would say the difference is that the partitive ones are like generally speaking, "Do you have a girlfriend overall." The second one (uutta tyttöystävää) you might use if you know that someone has broken up lately and you ask them if they have found a new girlfriend yet.
And the nominative one is a bit more specific. The only place I could imagine the first one being used is when someone mentions a girlfriend and it would come as a surprise to the other one: "Onko sinulla tyttöystävä???" maybe something you wouldn't have believed of that person or at least not yet. Same with the "uusi tyttöystävä", maybe someone recently broke up and you heard or saw them with the new girlfriend and you're asking them if it's true, did they (already) get a new one.