YOU SAID:

There was a cat eating my dog

INTO JAPANESE

私の犬を食べる猫がいた

BACK INTO ENGLISH

Cats that eat my dog was

INTO JAPANESE

私の犬を食べる猫だった

BACK INTO ENGLISH

Was my dog eat cat

INTO JAPANESE

私の犬は猫を食べていた

BACK INTO ENGLISH

My dog had eaten cat

INTO JAPANESE

私の犬は猫を食べていた

BACK INTO ENGLISH

My dog had eaten cat