Learn Finnish: Vanha perinne on tärkeä virstanpylväs. - An old tradition is an important milestone.

It is made up of virsta (an old unit of distance, similar to a mile or a Russian verst) in the genitive case (virstan) and pylväs (pillar or pole). Literally meaning a "mile's pillar," it translates perfectly to "milestone" in English.

How does the word perinne (tradition) change when we add case endings?

Perinne is an "e-stem" word (words ending in -e). When you add a case ending, the stem gets an extra -e-. For example, the genitive (whose tradition?) is perinteen and the inessive (in the tradition) is perinteessä. The partitive is formed by adding -ttä: perinnettä.

How do you find the stem to decline virstanpylväs?

In compound words, only the final part changes. Pylväs belongs to the word type ending in -as/-äs. To find the stem, drop the -s and lengthen the vowel: pylvää-. So the genitive is virstanpylvään (of the milestone). For the partitive case, the ending -tä is attached directly to the basic form: virstanpylvästä.

Why is the second half of the sentence (tärkeä virstanpylväs) in the nominative case rather than the partitive?

This is a standard "A = B" sentence (A is B). Because the subject A (perinne) is being equated to a singular, countable noun phrase B (tärkeä virstanpylväs), both sides stay in the basic nominative case. You would only use the partitive for B if A was an uncountable mass noun or an abstract concept being described by an adjective (like "Perinne on tärkeää" - Tradition is important).