Learn Finnish: Täällä ei keksitä myyttejä, vaan tutkitaan todellisuutta. - Myths are not invented here, but rather reality is studied.

Vaan is used specifically after a negative statement to mean "but rather" or "instead". Because the first clause establishes what is not done (ei keksitä), vaan is the right word to introduce the contrasting truth.

What forms are the verbs keksitä and tutkitaan in, and why do they look different?

They are both in the passive voice, used for actions done by an unspecified group (like "are invented" or "are studied"). However, ei keksitä is the negative present passive (which drops the final ending), while tutkitaan is the standard affirmative present passive.

Why is myyttejä in the partitive case?

It is the object of the negative verb phrase ei keksitä. A core rule in Finnish is that the object of a negative action must always be in the partitive case. Here, it is the partitive plural.

If the second clause is positive, why is todellisuutta also in the partitive case?

Because the verb tutkia (to study or research) is a partitive verb. Studying is an ongoing, continuous process rather than an action with a clear endpoint, so its object takes the partitive case even in an affirmative sentence.