Learn Italian: Quello specchio è rotto.

The word for "that" changes depending on the letters that start the next word, just like the word "the" does. Because specchio starts with an "s" followed by a consonant (p), it takes the article lo (lo specchio). For the exact same reason, "that" becomes quello here instead of the standard quel.

How would I say "Those mirrors are broken" in the plural?

You would say Quegli specchi sono rotti. Because quello acts like the definite article, it changes to quegli for masculine plural words starting with "s + consonant" (just like lo changes to gli). The verb and the adjective rotto also change to match the plural subject.

Does "quello" always change its ending before a noun?

Yes! When used right before a noun (as an adjective), quello shape-shifts. Before a masculine noun starting with a normal consonant, it is quel (quel libro). Before a vowel, it is quell' (quell'albero). It only becomes quello before nouns starting with tricky sounds like "s + consonant", "z", or "ps".

Why does "rotto" end in "-o" here?