Learn Finnish: Todiste tutkittiin nopeasti. - The evidence was examined quickly.

The easiest way to form the past passive is to start with the present passive. The present passive of tutkia (to examine) is tutkitaan (is examined). To make it past tense, simply change the -taan ending to -ttiin, giving you tutkittiin (was examined).

Why is todiste in its basic dictionary form (nominative)?

This is a special rule for the passive voice! In a normal active sentence, a total object usually takes the -n ending (like todisteen). However, when the action is done in the passive voice, the total object drops that ending and stays in the basic nominative form.

In English, "evidence" is usually an uncountable word. Is todiste singular or plural here?

It is strictly singular, referring to one specific piece of evidence or proof. If the police had examined multiple pieces of evidence, you would use the plural form instead: todisteet tutkittiin.

How does the adverb nopeasti (quickly) get its ending?

The base adjective is nopea (quick or fast). In Finnish, you can turn most adjectives into adverbs—which is exactly like adding "-ly" in English—by adding the ending -sti to the word. Therefore, nopea becomes nopeasti.