_______ a certain doubt among to the students as to the necessity of the work

(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 语法不对.There existed ...那我问下they have / has 是不是只有完成时才可以用不是，they have/ he has/she has可以单独用，也可以接完成时态动词，比如They have breakfast at 6 a.m. every morningThey have already finished breakfast.那这地方为什么不能用呢问题不是什么地方不能用，这样问等于本末倒置。they have 的意思是1. 他们有，他们做2. 他们做了，他们做好了，。。。后面加什么要看你要表达什么。上面我提供了两个例句：1.They have breakfast at 6 a.m. every morning他们每天早上6点吃早餐2. They have already finished breakfast. 他们吃完了早餐如果你想表达的是第一句的含义，那你就不能用完成时。