The boy was heard to sing a nursery rhyme in his room. 這一句男孩主動唱兒歌，主動不是應該用ing(singing) 嗎？主動為何卻用 to V？請問這裡有特殊的英文文法嗎？謝謝。?

1.使役動詞作被動式使用，其後動詞可接動名詞或不定詞，但以接動名詞比較符美語主流習慣
使役動詞hear，see被動:
(1).The boy was heard singing a nursery rhyme in his room. =
The boy was heard to sing a nursery rhyme in his room
(2). He is often seen reading comic books in class. =
He is often seen to read comic books in class.
2. 使役動詞作主動使用時，其後動詞接不定詞為強調事實，接動名詞
為強調動作
(1). I saw him crossing the street. (強調動作—我看到他正在過馬路)
(2). I saw him cross the street. (強調事實—我看到他有過馬路這件事)

使役動詞後