YOU SAID:

I forgot to fill my canteen with coffee.

INTO JAPANESE

私は食堂にコーヒーを入れるのを忘れた。

BACK INTO ENGLISH

I forgot to make coffee in the dining room.

INTO JAPANESE

私はダイニングルームでコーヒーを入れ忘れました。

BACK INTO ENGLISH

I forgot to make coffee in the dining room.