Learn Finnish: Uunissa on painava paistinpannu. - There is a heavy frying pan in the oven.

This is an existential sentence, which Finnish uses to say 'there is' or 'there are'. These sentences almost always start with the location. Because Finnish doesn't have articles like 'a' or 'the', word order does that job: starting with the location makes it 'There is A heavy frying pan in the oven.' If you started with the subject (Painava paistinpannu on uunissa), it would change the meaning to 'THE heavy frying pan is in the oven.'

The word painava looks like a verb participle. Is it derived from a verb?

Exactly! It is the present active participle (the -va form) of the verb painaa, which means 'to weigh' or 'to press'. Over time, this participle literally meaning 'weighing' or 'pressing' became the standard Finnish adjective for 'heavy'.

What is the function of the -n- in the middle of the compound word paistinpannu?

That is the genitive (possession) marker. The word is made up of paisti (a roast or joint of meat) and pannu (pan). By putting the first word in the genitive case (paistin), it literally translates to 'a roast's pan', which is the standard Finnish word for a frying pan.

If there were multiple pans, would the verb on change to the plural ovat?