YOU SAID:

i once ate an old man

INTO JAPANESE

私はかつて老人を食べたことがあります

BACK INTO ENGLISH

I once ate an old man

INTO JAPANESE

老人を食べたことがある

BACK INTO ENGLISH

have eaten old people

INTO JAPANESE

老人を食べた

BACK INTO ENGLISH

ate the old man

INTO JAPANESE

老人を食べた

BACK INTO ENGLISH

ate the old man