have her hair cut 中,可以不用 be.而 let her hair be cut 中,需要用 be.
雖然 have 和 let 都是使役動詞,但是,have sth.do 的用法,是使用 have 表示讓他人為你做某事,比如：i'd like to have my hair cut,my watch repaied,my room cleaned,都是讓別人為你剪頭發(fā),修表,打掃房間.這里,使用 have + 賓語 + 不帶 to的不定式做賓補(bǔ).賓語 hair,watch,room 都是動作的接受者,賓補(bǔ)是那個動作.換句話說,have sth do 本身帶有被動的含義.
而 let 不同,它構(gòu)成的 let + 賓語 + 賓補(bǔ),沒有被動的含義.如果要表示被動的概念,賓補(bǔ)部分用 be done.舉例：主動語態(tài)：I let him go,Jane's father let her see me in hospital.Don't let him die!被動語態(tài)：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l(wèi)et sth done 的形式嗎Let sth. be done 才正確。