Learn Icelandic: Diskurinn er skemmdur en mjög verðmætur. - The plate is damaged but very valuable.

To use an adjective independently like a noun (meaning "the [adjective] one"), you use its weak form, usually together with a word like þessi (this) or sá (that/the). Because diskur is a masculine noun, the weak form of skemmdur is skemmdi. So "the damaged one" would be sá skemmdi or þessi skemmdi.

Why do skemmdur and verðmætur use the strong adjective endings here?

Even though the noun diskurinn is definite (it has the -inn ending meaning "the"), the adjectives come after the verb er (is). Adjectives that follow the verb to describe the subject always use the strong form. They both end in -ur to match the masculine, singular noun diskur.

I sometimes see heldur used for "but". Why is en used in this sentence?

En is the standard word for "but" when contrasting two facts, like being damaged but valuable. You only use heldur (which means "but rather") when you are directly contradicting a negative statement, such as "The plate is not perfect, but rather damaged."

Verðmætur looks like a compound word. What are its parts?

It is made up of the noun verð (price, value) and the adjective mætur (worthy, highly regarded). Combined, they create the adjective verðmætur (valuable).