Final answer:
In a structure like 'neither/nor', the verb takes the form of the noun closest to it.
Step-by-step explanation:
When using a structure like 'neither/nor', the verb takes the form of the noun closest to it. In the example sentence 'Neither the students nor the teachers have proposed a field trip,' the verb 'have' agrees with the closest noun, which is 'teachers.' Therefore, the correct form of the verb is plural. This is because two or more subjects joined by 'neither/nor' are treated as a compound subject and take a plural verb in most sentences.