YOU SAID:

I once saw a cow on a field, who ate brown grass.

INTO JAPANESE

私はかつて野原で茶色の草を食べていた牛を見た.

BACK INTO ENGLISH

I once saw a cow eating brown grass in a field.

INTO JAPANESE

牛が野原で茶色の草を食べているのを見たことがあります。

BACK INTO ENGLISH

I have seen cows eating brown grass in the fields.

INTO JAPANESE

野原で牛が茶色い草を食べているのを見たことがあります。

BACK INTO ENGLISH

I have seen cows eating brown grass in the fields.