One phenomenon, two phenomena. Greek neuter nouns that end in -on change to -a in the plural.
You need to know this because the word phenomenon stubbornly refuses to go completely native. No one has the courage to write “phenomenons” as the plural. But because few of us know classical Greek, writers often get it backwards, taking “phenomena” as the singular. The Editor has even seen overzealous journalists putting a snooty “[sic]” after the correct singular use of “phenomenon” in a quotation.
Once again, as long as phenomenon clings to its Greek heritage, phenomenon is the singular and phenomena is the plural. When it at last decides to apply for English citizenship, the plural will be phenomenons.