Which is the difference of (nel and nello)? And how I should to use it?
Because in one excercise I put nel zucchero, and Duolingo put me (nello zucchero).
Masculine words beginning with z, gn, pn, ps, x or s + any consonant take the definite article lo before, so you should use nello, uno, allo, dello, dallo, collo, sullo and quello before them.
lo is the article used for words that start with 's' and 'z,' so if you said "The sugar," it would be "lo zucchero." Now as will many of the prepositions, the definite article blends with the preposition, so "nel" would be fine for masculine nouns, but "zucchero" starts with a "z," so you would instead say "nello zucchero" (Your question might have already been answered, but here's one anyway just in case)