The idea that it is only singular is a myth of unknown origin that seems to have emerged in the nineteenth century. If it appears to you as a singular in the context, use a singular; If it appears as a plural, use a plural. Both are acceptable beyond serious criticism. If none of them clearly means “not one,” a singular verb follows. The theme of the first part of the compound sentence is “Rebecca`s preference”. It is a singular subject. Therefore, the corresponding verb must be “is” and not “are”. 3. Look for the real sentence theme and choose a verb that matches that. The subject-verb compliance rules apply to all personnel pronouns except I and U which, although SINGULAR, require plural forms of verbs. Instead, the subject comes in this kind of sentence after the verb, so you have to look for it for the verb. In this sentence, the verb does not correspond to the subject. The theme “professor” is singular (“as well as some of the administrative staff” is a modifying expression and does not count), so the correct verb should be “was” and not “were”. Rule 2.
Two singular subjects, which are connected by or by or, or, or, or not, neither/nor connected, require a singular verb. However, a prepositional sentence inserted between the subject and the verb sometimes complicates the agreement. 4. Think about the indefinite pronoun exception that is taken into account in section 3.5, p.18: some, all, none, all and most. The number of these words is influenced by a prepositional sentence between the subject and the verb. The word that exists, a contraction from there, leads to bad habits in informal sentences as there are many people here today because it is simpler, “there are” than “there are”. Make sure you never use a plural subject.