Why does Spanish use double negatives?

Because Spanish negation is a grammatical agreement system, not a logical one. When a negative word (nada, nadie, nunca, ningún) follows the verb, the verb still needs no in front of it: no tengo nada (I have nothing, literally I do not have nothing). This is required, not optional. The double negative is only avoided when the negative word goes before the verb: nadie vino (nobody came) drops the no. English prescribes one negative per clause; Spanish prescribes agreement between the verb and any post-verbal negative word.

What is the difference between no and ninguno?

No is the general negation marker that sits before the verb (no quiero). Ninguno is a negative quantifier meaning none / not any, and it agrees with the noun it modifies. Ningún libro (no book), ninguna casa (no house). Ninguno almost always appears with no in front of the verb when it follows: no tengo ningún problema. It also shortens to ningún before a masculine singular noun, the same way uno shortens to un.

Spanish Negation

To negate any Spanish sentence, put no directly before the verb. That's the entire baseline rule.

Hablo español. (I speak Spanish.) → No hablo español. (I don't speak Spanish.)
Tengo tiempo. (I have time.) → No tengo tiempo. (I don't have time.)
María vino. (Maria came.) → María no vino. (Maria didn't come.)

No auxiliary verb. No do, does, did. The negation just sits in front of the verb, and the verb form doesn't change.

Position with object pronouns

If there are object pronouns, the no goes before them, not between them and the verb.

No lo veo. (I don't see it.) - not "lo no veo"
No me lo dijo. (He didn't tell it to me.) - not "me lo no dijo"
No se lavó las manos. (He didn't wash his hands.)

The cluster no + pronouns + verb stays together as a unit.

The negative words

Spanish has a set of dedicated negative words that pair with no in double-negative constructions.

Negative word	English
nada	nothing
nadie	nobody, no one
nunca / jamás	never
ninguno/a	none, not any
tampoco	neither, nor
ni... ni	neither... nor

The double-negative rule

When any of these negative words follows the verb, the verb still needs no in front of it. This is required, not optional.

No tengo nada. (I have nothing. Literally: I don't have nothing.)
No vino nadie. (Nobody came.)
No lo he visto nunca. (I have never seen it.)
No quiero ninguna. (I don't want any.)
No me gusta tampoco. (I don't like it either.)

This sounds wrong to English ears, where double negation is prescribed against. In Spanish it's required - the verb and the negative word agree, and stripping the no leaves the sentence broken.

The single-negative alternative

If the negative word goes before the verb instead of after, the no drops. Both orders are grammatical; the pre-verbal version is slightly more emphatic.

Nadie vino. (Nobody came.) - emphatic, single negative
No vino nadie. (Nobody came.) - neutral, double negative
Nunca lo he visto. (I have never seen it.) - emphatic
No lo he visto nunca. (I have never seen it.) - neutral

Pick whichever sounds right in the moment. Native speakers use both, and the meaning is the same.

Worked examples

No quiero nada. (I don't want anything.)
Nunca he estado en París. (I have never been to Paris.)
No conozco a nadie aquí. (I don't know anybody here.)
Ninguno de mis amigos habla inglés. (None of my friends speaks English.)
No me gusta el café ni el té. (I don't like coffee or tea.)
Yo tampoco. (Me neither.)

Common mistakes English speakers make

The big one is dropping the no in double-negative constructions: writing tengo nada instead of no tengo nada. The English-trained instinct to avoid double negatives is wrong here. The second is treating nadie and nada as English anybody and anything in positive sentences - they only work in negative contexts. For "is there anybody here?", you say ¿hay alguien aquí?, not ¿hay nadie aquí?.