Learn Finnish: Tämä uskomus perustuu siihen, että teoria lienee ollut oikein. - This belief is based on the fact that the theory probably was right.

This is the perfect tense of the potential mood, which is the focus of this lesson. It combines lienee, the present potential form of olla (meaning "is probably"), with the active past participle ollut. Together, they shift the probability into the past, translating to "probably has been" or "probably was".

Why do we need the word siihen right before the että clause?

The verb perustua (to be based on) always requires its target to be in the illative case (the "into" case). Because you cannot attach an että clause directly to a verb that demands a specific case, Finnish uses the pronoun se in the required illative case (siihen) as a grammatical bridge. Literally, the sentence says "is based on it, that...".

What is the difference between uskomus and the simpler word usko?

Both come from the verb uskoa (to believe). Usko generally refers to a broader "faith" or general belief (such as religious faith or having faith in someone's abilities). Adding the suffix -mus creates uskomus, which refers to a specific, discrete assumption, conviction, or myth that people hold as true.

Why use the adverb oikein instead of the adjective oikea for "right"?