he was heard singing=he was heard to sing?

问题描述:

he was heard singing=he was heard to sing?
dictionary tells us like this,
I don't know why

I am good at English.Let me tell you why:hear这样的词有两种结构,hear sb doing sth或 hear sb do sth he was heard singing=he was heard to sing?这两种说法都对,分别是上面两者的被动形式...