YOU SAID:

I did not realise this was a sad occasion.

INTO JAPANESE

私はこれが悲しい出来事だとは知りませんでした。

BACK INTO ENGLISH

I did not know this was a sad event.

INTO JAPANESE

これが悲しい出来事だとは知りませんでした。

BACK INTO ENGLISH

I did not know this was a sad event.