YOU SAID:

i once saw a great big whale

INTO JAPANESE

私はかつて大きなクジラを見ました

BACK INTO ENGLISH

I once saw a big whale

INTO JAPANESE

大きなクジラを見たことがあります

BACK INTO ENGLISH

I've seen a big whale

INTO JAPANESE

大きなクジラを見た

BACK INTO ENGLISH

I saw a big whale

INTO JAPANESE

大きなクジラを見ました

BACK INTO ENGLISH

I saw a big whale