YOU SAID:

i forgot my cat is lunch

INTO JAPANESE

猫が昼食であることを忘れました

BACK INTO ENGLISH

I forgot that the cat is lunch

INTO JAPANESE

猫がお弁当であることを忘れました

BACK INTO ENGLISH

I forgot that the cat is a lunch box

INTO JAPANESE

猫がお弁当であることを忘れてしまいました

BACK INTO ENGLISH

I forgot that the cat is a lunch box