Etymology and plural form
'Referendum' is the gerundive form of the Latin verb refero, literally "to carry back" (from the verb fero, "to bear, bring, carry" plus the inseparable prefix re-, here meaning "back"). As a gerundive is an adjective, not a noun, it cannot be used alone in Latin, and must be contained within a context attached to a noun such as Propositum quod referendum est populo, "A proposal which must be carried back to the people". The addition of the verb sum (3rd person singular, est) to a gerundive, denotes the idea of necessity or compulsion, that which "must" be done, rather than that which is "fit for" doing. Its use as a noun in English is not considered a strictly grammatical usage of a foreign word, but is rather a freshly coined English noun, which follows English grammatical usage, not Latin grammatical usage. This determines the form of the plural in English, which according to English grammar should be "referendums". The use of "referenda" as a plural form in English (treating it as a Latin word and attempting to apply to it the rules of Latin grammar) is unsupportable according to the rules of both Latin and English grammar. The use of "referenda" as a plural form is posited hypothetically as either a gerund or a gerundive by the Oxford English Dictionary, which rules out such usage in both cases as follows:
Referendums is logically preferable as a plural form meaning 'ballots on one issue' (as a Latin gerund, referendum has no plural). The Latin plural gerundive 'referenda', meaning 'things to be referred', necessarily connotes a plurality of issues.
It is closely related to agenda, "those matters which must be driven forward", from ago, to drive (cattle); and memorandum, "that matter which must be remembered", from memoro, to call to mind, corrigenda, from rego, to rule, make straight, those things which must be made straight (corrected), etc.