Like Amparo said. Now, it's perhaps interesting to note that you would be perfectly correct to use "es" if you were speaking Portuguese, French, or Latin. Example in European Portuguese "(Tu) no és uma pessoa negativa." Latin: "(Tu) non es persona negativa" or, more typically "Persona negativa non es."

Yes, you are right! Tú no eres una persona negativa. "Eres" is verb "ser" for second person singular. Verbo SER: yo soy, tú eres, él es, nosotros somos, vosotros sois, ellos son.

Traditionally is explained the difference between SER (defines a QUALITY, ATTRIBUTE) and ESTAR (defines a STATUS). "Timmy es alegre" (generally, normally) or "Timmy está alegre" (at this point in time). Even so you should keep in mind that the attributes of the people or things that define SER expresses and defines CHARACTERISTICS, regardless of whether they are permanent or temporary. Thereby, "The child is small (though certainly he's going to grow up).