Greek-derived nouns ending in -on change to -a in the plural form.
Upon is regular. It follows the Greek -a rule.
Use plural verbs: "The upa are ready" (not "is ready").
Yes. The plural upa is standard across all English variants.
The word upon follows the standard English pluralization rule. When forming the plural, upon becomes upa. This follows the Greek -a pattern, which is one of the most common plural rules in English.
Understanding why upon becomes upa helps with spelling and pronunciation. Greek-derived nouns ending in -on change to -a in the plural form.
When using upa in writing, remember that it functions as a plural noun in sentences. It pairs with plural verbs and pronouns. For example: "The upa are ready" (not "is ready"). This subject-verb agreement is essential for grammatical correctness.