I opened the cupboard door and then stood in front of it pet

是主语“I”的补语,主语和主补,宾语和宾补,都是逻辑上的主谓关系.如：
The sun is burning (hot).
Anderson came back to his hometown (a prince of fairy tales).
petrified 是petrify的过去分词形式,但分词都有形容词特性,这里应该当做是形容词,“吓呆了”的意思.