"Tu îi dai acelei fete o felie de tort."
Translation:You give that girl a slice of cake.
In this scenario, acelei fete is the dative form. Plural would be acelor fete.
For masculine things, it would be acelui băiat for singular and acelor băieți for plural.
Romanian is strange.
How do you know when to use the dative form? Like can't you say, "tu ii dai fata aia o felie de tort"?
It's dative. You can get the conjugation/declination table on the dex website or app: https://dexonline.ro/intrare/fat%C4%83/20337#paradigmTab