一、不定式1、在感观动词feel,hear,listen to,look at,notice,see,observe,watch等和使役动词make,let,have等词后的补足语中,不定式不带。但是这些句子如果变成被动结构时,就必须带。例如:We often hear him sing English songs.He is often heard to sing English songs.2、动词不定式在介词but,except,besides后面时,如果这些介词前 First, the infinitive 1, in the sense of the verb feel, hear, listen to, look at, notice, see, observe, watch, and make verbs make, let, have and other words complement word, infinitive without. However, if these sentences become passive structures, they must be taken. For example: We often hear him sing English songs.He is often heard to sing English songs.2, Verb infinitives after the preposition but, except, except, if these prepositions