Formal:
* Jestem smutny/smutna. (I am sad) This is the most common and literal translation.
Informal:
* Jestem przygnębiony/przygnębiona. (I am depressed/down) This is a slightly stronger expression.
* Jestem smutny/smutna. (I am sad) This is also used informally.
* Jestem zły/zła. (I am bad/evil) This is a more casual and playful way to say "I'm sad", but it is used to express a feeling of melancholy or disappointment.
Other options:
* Czuję się źle. (I feel bad) This is a more general expression of feeling unwell, but it can also be used to express sadness.
* Mam zły humor. (I am in a bad mood) This is a more colloquial way to say "I'm sad".
Note:
* Smutny/smutna are the masculine and feminine forms of the adjective "sad".
* You can also use the word smutek (sadness) instead of smutny/smutna, but it is less common to say "I am sadness" in Polish.
The best way to choose the right phrase depends on the specific situation and the relationship between the speakers.