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
That didn't even make that much sense in English.
