YOU SAID:

There never was a cat,

INTO JAPANESE

猫はなかった

BACK INTO ENGLISH

The cat was not

INTO JAPANESE

猫ではなかった

BACK INTO ENGLISH

Didn't cat

INTO JAPANESE

猫でした。

BACK INTO ENGLISH

It was a cat.

INTO JAPANESE

猫だった。

BACK INTO ENGLISH

It was a cat.