"The girl has a sandwich."
Translation:Fata are un sandvici.
Those lessons are weird. I see many people encounter them and most of them are wrong. The indefinite article is always used, especially in it's singular form. You can drop it in plural though, pretty much similar to english:
Fata are un sandvici - The girl has a sandwich
Fata are (niște) sandiviciuri - The girl has (some) sandwiches.