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

he was heard singing= someone heard him singing 强调唱歌动作与听到这个动作同时发生
he was heard to sing - someone heard him sing 强调“听到他唱歌来着”，唱完了

which dictionary? for spoken English or written ?如果是介绍口语的书。口语语法要求很松的。这样约等于是可以的。如果是介绍书面语的书，这样说应该是不妥的。
甚至严格来说，这样是错的he was heard singing=he was heard to sing。
he was heard singing.的主动句是 sb heard him singing.
he was heard to sing的主动句是sb heard him sing.
两句话有含义上的差别。以hear 为例子
do是听见做某事 （强调听见了全过程）
I hear him sing.我听见他唱歌。
doing是听见正在做某事（也许只是听见他唱了一句话而已）
I hear him singing.我听见他正在唱歌

有heard to 这种用法嘛

不相等
be heard do/doing

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...

不相等的
he was heard singing是错误的
hear sb do sth是主动语态，听见某人做某事
在变为被动语态时
是sb was heard to do sth
某人被听见做某事