(There existed) a certain doubt among the students as to the necessity of the work.There existed a certain doubt和There was a certain doubt有相同意思.It existed a certain doubt 語法不對(duì).There existed ...那我問下they have / has 是不是只有完成時(shí)才可以用不是，they have/ he has/she has可以單獨(dú)用，也可以接完成時(shí)態(tài)動(dòng)詞，比如They have breakfast at 6 a.m. every morningThey have already finished breakfast.那這地方為什么不能用呢問題不是什么地方不能用，這樣問等于本末倒置。they have 的意思是1. 他們有，他們做2. 他們做了，他們做好了，。。。后面加什么要看你要表達(dá)什么。上面我提供了兩個(gè)例句：1.They have breakfast at 6 a.m. every morning他們每天早上6點(diǎn)吃早餐2. They have already finished breakfast. 他們吃完了早餐如果你想表達(dá)的是第一句的含義，那你就不能用完成時(shí)。