Formal:
* Soltero/a: This is the most common and formal translation for "single" and can be used for both men and women.
* Sin pareja: This literally means "without a partner" and is also a formal option.
Informal:
* Solterón/a: This is a more informal term that can be used for someone who is single and enjoys their independence.
* Libre: This means "free" and can be used to describe someone who is single and enjoys their freedom.
Example sentences:
* Formal: "Soy soltero/a." (I am single.)
* Informal: "Estoy soltero/a y me gusta." (I am single and I like it.)
Note: In Spanish, gender agreement is important. You will need to use the masculine form "soltero" for a man and the feminine form "soltera" for a woman.