You certainly shouldn't go to that party tonight.

这句的意思是：今晚你当然不该去那个聚会。
表示原本不该的那个句子：You shouldn't have been to that party. 这里要在 shouldn't 后用完成时态，动词要用 have been to。

You shouldn't have gone to that party.你原本不该去参加那个聚会.certainly .adv.的确, (口语)当然, 行

You shouldn't have been to that party. 用have gone to 有语法问题,因为是虚拟语气,现在人已经不在聚会上了,用gone 代表一直没有回来.
certainly是个语气助词,"当然,肯定,明显",类似的还有absolutely等等

certainly:当然
You shouldn't have been to that party.
因你已回来,所以只能been, 不可用gone

You shouldn't have been to that party
虚拟语气，实际已经去了。