YOU SAID:

well i guess we can go to the party tonight, but i have to go tot he gym in the morning

INTO JAPANESE

今夜はパーティーに行けると思うけど、朝ジムに行かないといけない

BACK INTO ENGLISH

I think I can go to the party tonight, but I have to go to the gym in the morning.

INTO JAPANESE

今夜のパーティーには行けると思いますが、朝はジムに行かなければなりません。

BACK INTO ENGLISH

I think I can go to the party tonight, but I have to go to the gym in the morning.