YOU SAID:
He is going to the party with his girl
INTO JAPANESE
彼は彼の女の子とパーティーに行く
BACK INTO ENGLISH
He goes to a party with his girl
INTO JAPANESE
彼は彼の女の子とパーティーに行く
BACK INTO ENGLISH
He goes to a party with his girl
That didn't even make that much sense in English.