YOU SAID:

When I was walking outside, a red piano ate my dog!

INTO JAPANESE

私が外を歩いているとき、赤いピアノが私の犬を食べました！

BACK INTO ENGLISH

When I was walking outside, the red piano ate my dog!

INTO JAPANESE

私が外を歩いていたとき、赤いピアノが私の犬を食べました！

BACK INTO ENGLISH

When I was walking outside, the red piano ate my dog!