Learn Finnish: Me emme ymmärtäneet totuutta. - We did not understand the truth.

It is derived from the adjective tosi (true). Adjectives ending in -si typically have a stem ending in -te-, making the stem tote-. When you add the abstract noun suffix -uus, the final -e- is dropped, resulting in totuus.

How do you form the partitive case for nouns ending in "-uus"?

For words ending in -uus or -yys, you form the partitive by replacing the final -s with -tta or -ttä. This gives them a characteristic double -tt- in the partitive, which is why totuus becomes totuutta.

Why is "totuutta" in the partitive case here instead of the accusative?

In Finnish, the object of a negative sentence is always in the partitive case. Because the sentence uses the negative verb emme (we did not), the object totuus must take its partitive form, totuutta.

Why does the main verb take the form "ymmärtäneet" instead of "ymmärtänyt"?