Learn Polish: Wiadomo, że on zaginął wczoraj wieczorem. - It is known that he went missing yesterday evening.

Wiadomo is an impersonal predicate. This means it acts as the main verb of the sentence but doesn't take a subject (there is no 'it' or 'to' before it). Here, it pairs with the conjunction że (that) to introduce an established fact: 'It is known that...'

Could we use a question word like kiedy instead of że after wiadomo?

Yes, but the structure's purpose shifts slightly. With że, you state the entire following clause is a known fact. If you use a question word, you state that a specific detail is known, for example: Wiadomo, kiedy on zaginął (It is known when he went missing).

Why do we include the pronoun on here if Polish usually drops subjects?

While Polish is a pro-drop language, subject pronouns are often kept after conjunctions like że to clearly establish the subject of the new subordinate clause, especially if we are introducing a new topic or contrasting him with someone else.

What is the infinitive of zaginął and what should I watch out for in its past tense?

The infinitive is the perfective verb zaginąć (to go missing). Because it is a -nąć verb, you need to watch out for the vowel shifts in the past tense stem: it is zaginął for a masculine subject, but switches to an 'ę' for feminine (zaginęła) and virile/masculine personal (zaginęli).