By the way: are you aware of the so-called “impersonal verbs”? If so, something positive, if not, don't worry, because that won't prevent you from understanding the subject that now becomes evident.
Another issue, just remembering: as users of the language, we have to be aware that we are governed by a system, conventional to all of us and... therefore, it is only up to us to follow him, especially when it comes to formal situations of dialogue.
Well then, we go on analyzing the following statements:
They had candidates waiting for the results of the Vestibular.
Watch out! When trying to discover the subject of this sentence, know that it does not have it, given that it is a clause without a subject. What comes after the verb: The direct object: candidates waiting for the entrance exam results.
Therefore, if it is a question of a clause without a subject, whose underlying principle is the verb not to be inflected, why then is the verb of the example found in the plural? Simple, it consists of an error, a grammatical inconsistency.
Therefore, understand that even if the object is in the plural, the verb must necessarily remain in the third person singular, that is:
There were candidates waiting for the results of the Vestibular.
There were kids playing ball in the school yard.
There were people talking really loud. And so on...
But pay attention to an extremely important detail: when it comes to written language, always prefer to use the verb to have instead of the verb to have. So, reformulating we have:
There were candidates waiting for the results of the Vestibular.
There were kids playing ball in the school yard.
There were people talking really loud. And so on...
By Vânia Duarte
Graduated in Letters
Source: Brazil School - https://brasilescola.uol.com.br/gramatica/haver-ou-ter.htm