Learn Italian: Sento i cigolii dei vecchi lucernari. - I hear the creaks of the old skylights.

Both words come from singular masculine nouns ending in -io (cigolio and lucernario). The rule depends on where the stress falls in the singular form. In cigolio, the stress is on the i (cigolío), so it keeps that stressed i and adds the regular plural -i, resulting in cigolii. In lucernario, the stress is on the a (lucernàrio), so the unstressed -io simply becomes a single -i in the plural.

Is there a trick to knowing if the i in an -io noun is stressed or unstressed?

There isn't a perfect rule, but many nouns ending in -ìo that describe continuous or repetitive sounds (like cigolio for creaking, mormorio for murmuring, or fruscio for rustling) have the stress on the i. Most other everyday -io words (like lucernario, orologio, or viaggio) have an unstressed i and therefore take a single -i in the plural.

Why is there an h in vecchi? The singular is vecchio.

The h is required to preserve the hard 'k' sound of the singular vecchio. If you just dropped the -o and added an -i to get 'vecci', the c before an i would make a soft 'ch' sound (like in 'cheese'). Inserting the h makes it vecchi (pronounced 'vek-kee').

Why do we use dei here instead of degli?

Dei is the combination of di + i. Even though the noun phrase is vecchi lucernari, the article must match the word immediately following it, which is the adjective vecchi. Since vecchi starts with a simple consonant (v), it takes the plural article i, making the combined preposition dei. If the adjective started with a vowel or an 's+consonant' (like in degli strani lucernari), you would use degli.