I often hear people talk about a phenomenon, which refers to one thing or situation, when they need the plural of phenomenon—which is phenomena, referring to more than one thing or situation.
• Global warming is a potentially disastrous phenomenon.
• The phenomena that contribute to global warming are being studied extensively in hopes of avoiding worldwide catastrophes.
Another pair often misused are criteria (plural) and criterion (singular). If you have only one standard that must be met, you want criterion.
But here’s one you can stop worrying about: datum. That’s the singular of data. Today, data is used for both singular and plural. Why? Because common usage changes all languages. However, if you are using data as a plural, make your verb plural also:
• The scientific data are unequivocal that ocean temperatures are rising rapidly.