Learn Finnish: Äiti huolestui huonoista uutisista. - Mother got worried about the bad news.

In Finnish, verbs expressing a reaction or a change of emotion, like huolestua (to get worried), require the cause of the emotion to be in the elative case (-sta/-stä). Think of it as getting worried "from" the news.

Could I say Äiti oli huolissaan huonoista uutisista instead? What is the difference?

Yes, but the meaning changes slightly. Olla huolissaan describes a continuous state (to be worried), while the verb huolestua describes a change of state (to get or become worried). The past tense huolestui highlights her immediate reaction upon hearing the news.

How do we build the form uutisista from the dictionary word uutinen (news)?

Words ending in -nen change their ending to -s- before adding most case suffixes. To make it plural, we add the plural marker -i-, and finally the elative ending -sta. The stem uutise- becomes the plural uutisi-, and adding the case ending gives us uutisista.

Why does the adjective huono (bad) also take the -ista ending?