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.