As you know, you can use more than one negative marker in Spanish without switching polarity (or positive/negative-ness). Spanish actually gets pretty crazy when it comes to negation. Here's a reminder:
- Ella no quiere café.
- Ella no quiere nada.
- Él no quiere nada tampoco.
- Él no quiere nada nunca tampoco.
Those all pretty much mean the same thing—that nothing is wanted.
Standard American English is a little bit different: it takes the mathematical approach. By this, we mean that two negatives make a positive. It can get pretty convoluted.
Let's compare the English translations of the sentences above:
- She doesn't want coffee.
- She doesn't want anything.
- He doesn't want anything either.
- He never wants anything either.
In these examples, we only see the negative markers, not (or its contraction n't) and never. Be careful not to make the following mistakes:
- She doesn't want no coffee.
- She doesn't want nothing.
- He doesn't want nothing neither.
- He never wants nothing neither.
For the first two, if you add that extra negation, the meaning changes. Now, when you say, "She doesn't want no coffee," you're basically saying that she wants coffee. In the second one, you're saying that she wants something. (The second two are just plain incorrect in Standard American English, since "neither" can only be used with "nor.")
If we're being real, you can probably say these sentences and people will know what you mean in context, but it's not (say it with us) standard. Your English teacher will get mad, and we don't want that, do we?