动词不定式1．feel, hear, listen to 等感观动词和 have, let , make等使役动词后的补足语，在主动语态中，不定式不带to；在被动语态中，则必须带to。如：I often hear him sing the song. / He is often heard to sing the song.2. 不定式短语位于介词but, except, besides后时，如果
