to后不一定都是加动词原形的。 你给的句子He was very happy to_______,这里要用原形，be happy to do sth. 如果to作为介词，而不是短语中的固定搭配，后面一般跟的是名词或者人称代词 ，比如: He went to the supermarket last night. Please pick up the book and give it to me. 某些固定搭配还可以跟动名词，比如look forward to sth./doing sth. We are looking forward to your next visit. I am looking forward to going shopping with you. 你说的加第三人称单数的情况好像是没有的。。。不会因为句子的助于是he,后面就填第三人称单数，句中的系动词was已经起了这个功能。