Learn Italian: Nessuno vuole rubare la tua vecchia macchina.

When a negative word like nessuno (no one) comes before the verb as the subject of the sentence, you do not use non. You only use the double negative (putting non before the verb) when nessuno comes after the verb, like in Non voglio vedere nessuno (I don't want to see anyone).

What verb form do we use with "nessuno"? Is it always singular?

Yes, nessuno (no one / nobody) is always grammatically singular, just like in English. That's why it takes the third-person singular verb form vuole (he/she wants).

Why is it "la tua vecchia macchina" and not just "tua vecchia macchina"?

In Italian, possessive words like tua (your) almost always need a definite article (like il, la, i, or le) in front of them. The main exception is when talking about singular close family members (like mio padre), but for a car, you must include the la.

Why does the adjective "vecchia" come before the noun "macchina"? I thought adjectives go after the noun.