what a surprise to get a letter from you at last.请问to 是做什么成分

what a surprise to get a letter from you at last.请问to 是做什么成分
what a surprise to get a letter from you at last.请问to 是做什么成分

what a surprise to get a letter from you at last.请问to 是做什么成分
主语.
但是省掉了很多,等于 to get a letter from you at last is a big surprise.