Phenomenon and phenomena are the singular and plural forms of the same word. The difference is simple: use phenomenon for one event, fact, or occurrence, and use phenomena for more than one.
This pair often causes confusion because the plural form sounds formal, and some writers mistakenly use it for a single thing. A helpful check is to look at the rest of the sentence. If the sentence refers to one item, choose phenomenon. If it refers to several, choose phenomena.
- Correct: The northern lights are a natural phenomenon.
- Correct: Scientists studied several unusual phenomena in the atmosphere.
- Incorrect: This strange phenomena is hard to explain.
In that last sentence, the noun is singular, so phenomenon is the correct choice. Remember it this way: one phenomenon, many phenomena. Once you match the word to the number, the choice becomes easy.