Questions & Answers about La porta di vetro è rotta.
Why do we need the word di here?
Why is it just di vetro and not combined with an article, like del vetro?
When you use di to describe the material an object is made of, you just use the noun for the material on its own, without adding "the" (the definite article).
Why does rotta end in -a instead of -o like vetro?
What is the base verb for rotta?
It comes from the verb rompere (to break). The past participle is rotto (broken), which is irregular. Here it is used as an adjective, which is why its ending changes to match la porta.