Learn Finnish: Virheetön arvoesine on kallein. - A flawless valuable item is the most expensive.

It is formed by attaching the caritive suffix -tön (meaning 'without' or '-less') to the noun virhe (flaw or mistake). When adding -tön or -ton to a word, the final vowel of the basic word stem is always lengthened. Thus, virhe gets an extra e, becoming virheetön.

How does the word kallis turn into the superlative kallein in this sentence?

Words ending in -is (like kallis) have a vowel-stem ending in -ii- or -ei-. For kallis, the base stem is kallei-. When adding the superlative marker -in, the vowels merge and simplify, resulting in the form kallein (the most expensive).

Why is kallein in the nominative case here rather than the partitive?

Because the subject arvoesine (valuable item) is a countable, singular noun. When describing a countable noun with the verb olla (to be), the predicative adjective must be in the nominative case. If the subject were an uncountable mass noun, like kulta (gold), the adjective would take the partitive case, as in 'kulta on kalleinta'.

What are the components of the compound word arvoesine?