We have heard him sing改爲被動語態.He ____ ____ ____ ____ ____ by

題目：

We have heard him sing改爲被動語態.He ____ ____ ____ ____ ____ by us.
hear 後面不是不可以加to do 不定式嗎,請注意要填五個空！

解答：

has been heard to sing 被動語態時"to do"是要還原的