Could you correct the sentences of passive voice

[engee30]

(I think Engee might have invented a new word: passivize. )

Oh no, the verb passivize is widely used in linguistics. But personally speaking, I wish I had.
Anyway, thanks for that comment, RonBee.

[Pedroski]

I think something is not right with 1) and 4)

The passive form of an active sentence is formed by taking the object of the active sentence and making it the subject of a passive sentence. Even though it is the subject of the new sentence, it is still the recipient of the action of the verb. This is the definition of a passive sentence.

In 1) I heard him sing a song, the object is not 'him', but 'him sing a song'.
what
The correct passive form of the sentence is then:

His singing a song was heard by me. This is correct, because what I heard was his singing, not him.

Similarly in 4) They think that Mary can accept the new idea.

'That Mary can accept the new idea was thought by them.' is the correct passive rendition of 4)

To passivize 'Clean the window Mary' you need to know that 'Mary' here is an absolute phrase, unconnected with the sentence 'Clean the window.'

The passive is then The window should be cleaned, Mary.

[WUKEN]

The correct passive form of the sentence is then:

His singing a song was heard by me. This is correct, because what I heard was his singing, not him.

Thanks for all of your clarifications.

I am a bit confused about the sentences of passive voice by different versions.
All I am sure from everyone's input is that the passive voice of imperative mood is rarely used in real life although it's introduced in grammar books.
But how about
I heard him sing a song.?

I am wondering what the passive voice is.
May I have more replies?
Thanks a lot!

[RonBee]

Active voice: I heard him sing a song.

Passive voice: His singing (of the song) was heard by me.