have her hair cut 中,可以不用 be.而 let her hair be cut 中,需要用 be.
虽然 have 和 let 都是使役动词,但是,have sth.do 的用法,是使用 have 表示让他人为你做某事,比如:i'd like to have my hair cut,my watch repaied,my room cleaned,都是让别人为你剪头发,修表,打扫房间.这里,使用 have + 宾语 + 不带 to的不定式做宾补.宾语 hair,watch,room 都是动作的接受者,宾补是那个动作.换句话说,have sth do 本身带有被动的含义.
而 let 不同,它构成的 let + 宾语 + 宾补,没有被动的含义.如果要表示被动的概念,宾补部分用 be done.举例:主动语态:I let him go,Jane's father let her see me in hospital.Don't let him die!被动语态:John let his car be washed yesterday.Jane let her watch be repaired in the shop.We'll never let our old friends be forgotten.有let sth done 的形式吗Let sth. be done 才正确。