YOU SAID:

i forgot he was going out

INTO JAPANESE

彼が出かけるのを忘れた

BACK INTO ENGLISH

Forgot he went out

INTO JAPANESE

彼が出かけたことを忘れた

BACK INTO ENGLISH

I forgot that he went out

INTO JAPANESE

彼が出かけたことを忘れた

BACK INTO ENGLISH

I forgot that he went out