YOU SAID:

I once see a dragonfly

INTO JAPANESE

私はかつてトンボを見ました

BACK INTO ENGLISH

I used to see dragonfly

INTO JAPANESE

トンボを見たことがある

BACK INTO ENGLISH

I have seen a dragonfly

INTO JAPANESE

私はトンボを見ました

BACK INTO ENGLISH

I saw a dragonfly

INTO JAPANESE

トンボを見た

BACK INTO ENGLISH

I saw a dragonfly