带复合宾语的动词变被动时为什么不用加by

不是不用加by
,而是by
短语可省略（不是强调动作的执行者时）。如
1.
They
heard
her
sing
an
English
song
at
the
party.
他们在晚会上听到她唱了一首英语歌。
被动语态是：:She
was
heard
to
sing
an
English
song
(by
them
)
at
the
party.
（by
them
可省略。）
2.Jim
often
see
him
play
basketball
on
the
playground.
吉姆经常看见他在操场上打篮球。
被动语态是：
He
is
often
seen
to
play
basketball
by
Jim
on
the
playground
.(by
JIm
不可省略）
拓展：
含复合宾语的动词变成被动语态时,将宾语变为主语,宾语补足语变为主语补足语,但若谓语动词为listen
to,feel,hear,see,look
at,notice,make等而宾补不带to的不定式时,变被动应加上to.