Learn Swedish: Alla pengarna är stulna. - All the money is stolen.

In this sentence, the past participle is acting like an adjective describing the noun. Because pengarna (the money) is plural, the past participle must also take the plural form, changing from stulen to stulna.

Why is money treated as a plural word here, using alla and stulna?

Unlike in English where money is singular, the Swedish word pengar is plural. You can think of it like coins or funds. Because of this, any adjectives or participles attached to it must also be in their plural forms.

Why is it pengarna with a definite ending instead of just pengar?

When you want to say all of a specific thing, Swedish uses alla followed by the definite form of the noun. Alla pengarna means all the money (that specific money), whereas alla pengar would mean all money in the world generally.

What is the original verb that stulna comes from?

It comes from the irregular verb stjäla (to steal). Its base past participle form is stulen, which becomes stulna when describing a plural noun.