Final answer:
The subject-verb agreement in 'Either this generation or another keeps the fire of heroism alive' adheres to the rule requiring the verb to agree with the closer subject in cases of compound subjects joined by 'or'.
Step-by-step explanation:
The subject-verb agreement in the sentence 'Either this generation or another keeps the fire of heroism alive' is an example of agreement with compound subjects joined by or. When compound subjects are connected by or or nor, the verb should agree with the nearer subject. In this case, 'another' is closer to the verb 'keeps,' which is why the verb is singular. This is different from subjects joined by and, which typically require a plural verb unless they form a single idea or unit. Clear subject-verb agreement is crucial for creating rhetorically effective sentences that adhere to conventional grammar standards.