YOU SAID:

I just can't believe it, they forgot the onions.

INTO JAPANESE

信じられない、玉ねぎを忘れた。

BACK INTO ENGLISH

I can not believe it, I forgot the onions.

INTO JAPANESE

私はそれを信じることができない、私は玉ねぎを忘れた。

BACK INTO ENGLISH

I can not believe it, I forgot the onions.