Spanish does not normally mix negative and affirmative words. Unlike English, Spanish allows for the use of double, and triple negatives.

• Juan no necesita algo. (incorrect)
  Juan doesn't need anything. (correct)

• Juan no necesita nada. (correct)
  Juan doesn't need nothing. (incorrect)

• No tengo nada que hacer. (correct)
  I don't have nothing to do. (incorrect)

Let's look at some examples of sentences that use more than one negative expression:

• No tienes nada que hacer.
  You don't have anything to do.

• Ese niño no se lleva bien con nadie.
  That boy doesn't get along well with anyone.

• Gabriel nunca ha contribuido nada a la clase.
  Gabriel has never contributed anything to the class.

• Nunca debes hablar con ningún extraño.
  You should never speak to any stranger.

• ¿No tienes nada que decir?
  Don't you have anything to say?