Double negation. A common mistake that the spaniards usually do.
"No he estudiado nada"
"No quiero nada"
"Yo no lo sé tampoco"
"No veo nada"
"No tiene ninguna gracia"
In our language (spanish) it's very common to use the double negation to express something in negative like the examples above. Nevertheless, if you think about it, you'll realise that these sentences have no sense at all, because you are negating the negation, but we have accustomed to understand these sorts of sentences in our own way.
However, I would like you to help me with the matter, because the expressions above are expressions that I usually use in my way of thinking. I'll try to learn your advice so that I can use it like a reflex when I try to express something similar. Thanks a lot!