so可以用在否定句吗?

当然可以了.例如:She is not so pretty as you say.
It's not so difficult in fact.

当然可以
Mary is tired, so I don't ask her go shopping with me.

可以，如she's not so beautiful.她并不是那么漂亮，也可以说she is not that beautiful.

SO做所以讲时是可以用在否定句的，就像2楼说的，不过我觉得那个例子时态应该改称过去时比较好,毕竟是过去发生的事情嘛。
Mary was tired, so I didn't ask her go shopping with me.

of course!!!

可以