Learn Finnish: Me emme keksi valheita, vaan kerromme totuuden. - We do not invent lies, but rather we tell the truth.

While both mean "but," vaan is used specifically after a negative clause to mean "but rather" or "instead." Because the first half of the sentence is negative (emme keksi), and the second half provides the contrasting true alternative, we must use vaan.

Why is the first object (valheita) in the partitive case, but the second object (totuuden) in the genitive-accusative case?

This is a perfect example of how negation changes the object! The first clause is negative (emme keksi), and objects of negative sentences must always be in the partitive case (valheita). The second clause after vaan is affirmative (kerromme), so its complete object takes the normal genitive-looking accusative form (totuuden).

What is the base form of valheita, and how is this partitive plural formed?

The base word is valhe (a lie). For Finnish words ending in -e, you form the plural by taking the stem (which adds an extra -e), adding the plural marker -i-, and finally the partitive ending -ta. This creates valheita.

Do I need to repeat the pronoun me (we) after vaan?