YOU SAID:

the was once a cat sitting in a mat with a hat

INTO JAPANESE

かつては帽子をかぶってマットに座っていた猫でした

BACK INTO ENGLISH

I used to be a cat in a hat and sitting on a mat

INTO JAPANESE

帽子をかぶってマットの上に座る猫だった

BACK INTO ENGLISH

It was a cat wearing a hat and sitting on a mat

INTO JAPANESE

帽子をかぶってマットの上に座る猫でした

BACK INTO ENGLISH

It was a cat wearing a hat and sitting on a mat