Learn Finnish: On varma, että tämä on oikeutettu teko. - It is certain that this is a justified act.

In this sentence, the että clause acts as the subject. When the main clause adjective expresses a truth value or fact (like being certain or clear), standard Finnish requires the nominative case. If the adjective evaluated the nature of the action instead (like being important or difficult), you would use the partitive, such as in on tärkeää, että.

Will I hear native speakers say on varmaa, että with the partitive?

Yes, absolutely. In spoken language and casual writing, native speakers often use the partitive varmaa by analogy with evaluative adjectives. However, at the B2 level, you should know that standard written Finnish strictly prefers the nominative on varma, että.

How is the word oikeutettu formed?

It is the passive past participle (the TU-participle) of the verb oikeuttaa (to justify or to entitle). In this sentence, it functions completely as a regular adjective to modify teko.

Why doesn't the word teko take the partitive case?

Because the sub-clause tämä on oikeutettu teko is a standard 'A is B' sentence featuring a singular, countable noun. In these structures, the subject complement (oikeutettu teko) stays in the nominative case. You would only use the partitive if it were an uncountable mass noun, like in tämä on puhdasta vettä (this is clean water).