YOU SAID:

my dad left to get a gallon of milk and he didn't come back

INTO JAPANESE

父は牛乳を1ガロン取りに行ったのに戻ってこなかった

BACK INTO ENGLISH

My father went to get a gallon of milk and never came back.

INTO JAPANESE

父は牛乳を1ガロン取りに行ったまま戻ってきませんでした。

BACK INTO ENGLISH

My father went to get a gallon of milk and never came back.