YOU SAID:

Id go tomorrow but i have to help my underclassmen with a chinese event

INTO JAPANESE

明日行くけど、中国のイベントで後輩の手伝いをしなければならない

BACK INTO ENGLISH

I'm going tomorrow, but I have to help my junior at an event in China.

INTO JAPANESE

明日行くのですが、中国のイベントで後輩の手伝いをしなければなりません。

BACK INTO ENGLISH

I'm going tomorrow, but I have to help my junior at an event in China.