Learn Finnish: Puinen koriste on yhä ehjä. - The wooden decoration is still intact.

There isn't one here, but it's a great opportunity to contrast a noun with a participle! Koriste is a noun meaning "decoration". If you wanted to use a past passive participle as an adjective, you would use koristettu (decorated), from the verb koristaa (to decorate). For example, you might say puinen, koristettu laatikko (a wooden, decorated box).

Can I use vielä instead of yhä to mean "still"?

Yes, you can absolutely say on vielä ehjä. Both words mean "still". Yhä often emphasizes that a state has continued uninterrupted for a long time, sometimes despite expectations, whereas vielä is a bit more neutral and versatile. At this level, you can usually swap them without changing the meaning.

How is the word puinen (wooden) formed?

It is formed by taking the noun puu (wood or tree) and adding the suffix -nen. This is a very common way to create adjectives that describe what something is made of. Other common examples you might know are lasinen (made of glass) from lasi, and muovinen (made of plastic) from muovi.

What is the opposite of ehjä (intact) if I wanted to say the decoration is broken?