Question

We are going to his birthday party改为否定句。

Answer 1

We are going to his birthday party改为否定句是We are not going to his birthday party.
否定句改法：
1、看句中有无be动词，如有，直接在be动词后+ not。即：be动词(am、is、are、was、were)+not、
I am a teacher.我是个老师否定：I am not a teacher．我不是老师。
2、看句中有无情态动词，如有，直接在情态动词后+ not。即：情态动词(can、must、should)+not、
He can help me．他能帮助我。否定：He can't help me．他不能帮助我。
3、如上述二者都没有，就应用助动词+ not。即：助动词(do、does、did) + not
Put them over there．把他们放那边。否定：Don't put them over there．不要把他们放那边。

Answer 2

are后面加not即可。