We're going to have dinner when you're finished playing your game.
ーYou better study when you're finished that.
「それが終わったら勉強しなさいね」
You better ... when you're finished that. で「それが終わったら…しなさい」
ーWe're going to have dinner when you're finished playing your game.
「ゲームが終わったら夕飯食べるよ」」
We're going to ... when you're finished ~ing. で「〜するのが終わったら…するよ」
ご参考まで！
カナダ 
