YOU SAID:

I've heard of cows, but I've never herd of cows.

INTO JAPANESE

牛のことは聞いたことがありますが、牛の群れは一度もありません。

BACK INTO ENGLISH

I have heard about cows, but there has never been a herd of cows.

INTO JAPANESE

牛について聞いたことはありますが、牛の群れは一度もありませんでした。

BACK INTO ENGLISH

I have heard about cows, but there has never been a herd of cows.