YOU SAID:

once there was a cow. the cow ate pee.

INTO JAPANESE

牛がいたとき牛はおしっこを食べた。

BACK INTO ENGLISH

When the cows were there, they ate pee.

INTO JAPANESE

牛がそこにいるとき、彼らはおしっこを食べました。

BACK INTO ENGLISH

They ate pee when the cows were there.

INTO JAPANESE

牛がそこにいるとき彼らはおしっこを食べた。

BACK INTO ENGLISH

They ate pee when the cows were there.