把下列句子改为被动语态 I often hear Mary sing English songs at home.2.A policeman is running after a theif in the street.

Mary is often heard to sing songs at home
A thief is being run after by a policeman in the street

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以下供参考：
1. Songs sung by Mary was often heard at home. 家里经常听到玛丽的歌声。
难点就在于原句是一个连动句：我经常听到玛丽在家里唱歌。英语叫宾语从句。如果改成被动句，意味着要将整个宾语从句倒过来。一般而言，不要将人名放在被动句的句首，宜将从句中的宾语提出来，放在最前面。

2. A Thief is being runned by a policeman in the street。大街上，一名警察正在追赶一个小偷。
原句是典型的主谓宾结构，直接将宾语提出来，放在句首即可。注意after可以省略，因为be runned by 就是被追赶的意思。

Mary is ofen heard singing English sangs at home by me. 2. A theif is being run after by a policeman in the street.

1.Mary is often heard sing English songs at home(by me).
2.A theif is being run after by a policeman in the street.