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.

带复合宾语的句子变被动句原则上要找准宾语,把宾语放到主语的位置,把谓语动词的主动语态变为被动语态就可以了,强调主语就用by 连接主语,其他都在原来的位置.
He asked me to help him with his English.
I was asked to help him with his English.
The boss made them work ten hours a day .
They were made to work ten hours a day by the boss.
His mother keeps their room clean and tidy.
Their room is kept clean and tidy by his mother .（注意,宾补可以是 adj ,adv ,n ,to do ,doing ,done 等形式）
名词作宾补的主要有 call sb sth /name sb sth / make sb sth ,
eg ,We call him John.
They named the baby Jane.
He made London the base for his revolutionary work .