Learn Finnish: Uusi vuokrasopimus tiukkoine ehtoineen yllätti minut. - The new lease with its strict conditions surprised me.

This demonstrates the core rule of the comitative case. The adjective tiukka (strict) takes its plural stem tiukko- and adds the comitative ending -ine, becoming tiukkoine. The noun ehto (condition) takes its plural stem ehtoi-, adds -ne-, and must also take a possessive suffix. Since the lease owns the conditions, we use the third-person possessive suffix -en (its), making it ehtoineen.

Why doesn't the adjective tiukkoine have a possessive suffix like the noun does?

In the comitative case, modifier agreement is unique. While adjectives must agree with the noun by taking the -ine ending, they never take a possessive suffix. Only the head noun (ehtoineen) receives the possessive suffix at the very end of the word.

Why does the final 'a' change to 'o' when tiukka becomes tiukkoine?

The comitative case is always built on the plural stem, which is triggered by the plural marker -i- inside the -ine ending. For adjectives and nouns ending in -a, that vowel changes to -o- before the plural -i-. Therefore, tiukka becomes tiukko- before we add the -ine ending.

Why is the object minut instead of the partitive minua?