'Quel' vs 'quello'

If you have a masculine noun, the definite article (in singular) is either il or l' or lo. L' if the noun begins with a vowel (e.g. l'aeromobile), and lo if it begins with:

• s + consonant (e.g. lo studente)
• z (e.g. lo zio)
• y (e.g. lo yogurt)
• ps (e.g. lo psicologo)
• gn (e.g. lo gnomo)
• pn (e.g. lo pneumatico)

For the rest (the majority of nouns): il. Plural form of il is i, and plural form of l' and lo is gli.

If you have a feminine noun, it's either la or l'. The same rules, la for the majority of nouns and l' for those which begin with a vowel (e.g. l'automobile). The plural form of both is le.

There’s another lo rule, it’s i + vowel.
lo ione, lo iodio

And x.
lo xilofono, lo xilema

Lo