Mother said she was glad she never let her hair be cut.为什么用be cut 直接用cut不行吗?

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。