He is often heard to______(sing) songs in class. 括号里应怎样填,为什么?

填sing
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do和doing的两种用法
hear sb.doing sth.” 的意思是“听见某个动作的
一部分”,或者意为“听见这个动作正在进行”；
而“hear sb.do sth.”则表示“听到整个动作”,
或者意为“听到这个动作完成了”.
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在feel,hear,listen to,look at,notice,observe,see,watch,have,let,make等词后的补足语中,不定式不带to.但是这些句子如果变成被动结构时,就必须带to.例如：I often hear him sing the song.He is often heard to sing the song.
注意：不定式动词在介词but,except,besides后面时,如果这些介词之前有行为动词do的各种形式,那么,这些介词后的不定式不带to,否则要带to.如：
She could do nothing but cry.What do you like to do besides swim?
I have no choice but to go.