YOU SAID:

my cat was eating tuna and the the chicken ate corn

INTO JAPANESE

私の猫はマグロを食べていて、鶏はトウモロコシを食べた

BACK INTO ENGLISH

My cat was eating tuna and the chickens were eating corn

INTO JAPANESE

私の猫はマグロを食べていて、ニワトリはトウモロコシを食べていた

BACK INTO ENGLISH

My cat was eating tuna and the chickens were eating corn