改为被动语态,We ofen heard Mary sing in her bedroom.

Mary ofen sing was heard in her bedroom by we.
我们经常能听到MARY在她卧室唱歌。

Mary is often heard to sing in her bedroom.

Mary was ofen heard to sing in her bedroom by us .

Mary singing in her bedroom was often heard

Mary is often heard to sing in her bedroom .一定要加to 。因为在被动句中动词的不定式中to不能省略。

我们ofen听到玛丽唱歌，在她的卧室。

Mary's singing in her bedroom is often heard by us.

Mary is often heard to sing in her bedroom.
在主动语态中,使让动词(make,have,let)和感官动词(see,look,watch,notice,listen,hear,feel,find)后必省略to,但在被动语态中必须加上to

改被动为 Mary was often heard to sing in her bedroom (by us)
原句hear sb do sth中省略了to 改被动还原加to