楼主,
surprised在这里是形容词的词性,您可以去查字典验证!
另外,to作为动词不定式的符号的时候,后面都接动词原型.
surprise的用法如下：
1.I was surprised to find that!找到它让我感到吃惊
2.That was surpring!那真令人吃惊
3.It has surprised me a lot!它让我很吃惊
如果还有问题,欢迎讨论～