Here's why:
* "Or" indicates a choice. The verb needs to match the subject that's actually being chosen.
* Subject-verb agreement rules still apply. Singular subjects take singular verbs, and plural subjects take plural verbs.
Examples:
* "The cat or the dog is in the garden." (The verb "is" agrees with "dog", the subject closest to it.)
* "The teacher or the students are responsible for cleaning the classroom." (The verb "are" agrees with "students", the subject closest to it.)
Let me know if you'd like more examples!