Learn Italian: Nessuno ha comprato questo bello specchio di bronzo. - Nobody bought this beautiful bronze mirror.

When bello is placed before a noun, its ending changes based on the same rules as the definite articles (il, lo, la, etc.). Because specchio is a masculine word starting with an 's' plus a consonant, it takes the article lo (lo specchio). Therefore, the matching form for 'beautiful' is bello.

Doesn't Italian use double negatives? Why is there no non before the verb?

Italian does use double negatives, but only when the negative word comes after the verb. Because nessuno (nobody) is placed at the very beginning of the sentence, right before the verb ha comprato, you do not add non.

Would the sentence still be correct if I put bello after the noun, like specchio bello?

Yes, you could say questo specchio bello. When bello comes after the noun, it acts like a regular adjective and you don't have to worry about the special phonetic rules (like choosing between bel and bello). However, placing it before the noun is much more common for adjectives describing beauty.

Why does the sentence say di bronzo instead of just bronzo specchio?