YOU SAID:

I'm going to buy coffee when I go home today

INTO JAPANESE

今日は家に帰るとき、私はコーヒーを購入するつもりです

BACK INTO ENGLISH

When you go home, I'm going to buy a coffee today

INTO JAPANESE

あなたが家に帰るとき、私は今日のコーヒーを買いに行きますよ

BACK INTO ENGLISH

When you go home, I will go to buy a cup of coffee today

INTO JAPANESE

あなたが家に帰るとき、私は今日コーヒーを買いに行きます

BACK INTO ENGLISH

When you go home, I will go to buy a cup of coffee today