Adjective can be used, such as : glad, sad, worried, angry, pleased, delighted, disappointed, shocked, afraid, unable, willing, sure, anxious, able, eager, etc.
- I was sorry when/because I heard the bad news.
= I was sorry to hear the bad new.
- He is ready to go.
- I am glad to meet you.
- To understand this lesson is easy.
It is easy to understand this lesson.
* It is possible that he telephoned while we were out (= he may have telephoned)