YOU SAID:

I almost forgot the dog was celebrating his birthday today.

INTO JAPANESE

彼の誕生日だったのをほとんど忘れそうだった。

BACK INTO ENGLISH

I almost forgot that it was his birthday.

INTO JAPANESE

彼の誕生日だったのをほとんど忘れそうだった。

BACK INTO ENGLISH

I almost forgot that it was his birthday.