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Questions & Answers about Rikollinen pidätettiin tänään.
How do we build the past passive form pidätettiin from the dictionary verb?
Why is rikollinen in the basic nominative form, rather than an object case?
In the passive voice, a total object (meaning the action was fully completed on the person) takes the basic nominative form. If this were an active sentence like 'The police arrested the criminal', it would take the -n ending (rikollisen). But since it is passive, we use the basic form rikollinen.
Why does pidätettiin have a single 't' in the middle while the basic verb pidättää has double 'tt'?
Adding the passive ending closes the syllable, which triggers consonant gradation. The strong double 'tt' from the dictionary form pidättää weakens to a single 't' in the past passive form pidätettiin.
The word rikollinen ends in -nen, which often marks adjectives. Is it a noun or an adjective here?
It acts as a noun here. It comes from the word rikos (crime). In Finnish, words ending in -nen are often adjectives, but they can frequently be used as nouns to describe a person with that trait, just like 'a criminal' in English.