YOU SAID:

I am having a party tonight.

INTO JAPANESE

今夜はパーティーをしています。

BACK INTO ENGLISH

I'm having a party tonight.

INTO JAPANESE

今夜はパーティーをします。

BACK INTO ENGLISH

We're having a party tonight.

INTO JAPANESE

今夜はパーティーがあります。

BACK INTO ENGLISH

We have a party tonight.

INTO JAPANESE

今夜はパーティーがあります。

BACK INTO ENGLISH

We have a party tonight.