YOU SAID:

I have decided to not go to the party

INTO JAPANESE

私はパーティーに行かないことにしました

BACK INTO ENGLISH

I decided not to go to the party

INTO JAPANESE

彼はパーティーに行かないことに決めた。

BACK INTO ENGLISH

He decided not to go to the party.

INTO JAPANESE

彼はパーティーに行かないことに決めた。

BACK INTO ENGLISH

He decided not to go to the party.