Learn Finnish: Me emme väsyneet valoisaan kesään. - We did not get tired of the bright summer.

The verb väsyä (to get tired) has a strict rule, or rection: it always requires the illative case. While in English you get tired of something, in Finnish you get tired into it!

Why is the verb form väsyneet here instead of a regular past tense?

This is the negative past tense. To say we did not get tired, you use the negative verb emme alongside the plural past participle of the main verb. Because the subject is me (we), the ending is plural (-neet), making it väsyneet.

Why do the two words have slightly different illative endings, -aan and -ään?

This is a great reminder of vowel harmony. Both words are getting the standard illative ending, which means doubling the last vowel and adding n. However, they must match their own internal vowels. Valoisa has back vowels, so it becomes valoisaan. Kesä has front vowels, so it becomes kesään.

Why does the adjective valoisaan also have the illative ending?

In Finnish, adjectives must agree with the noun they describe. Because the verb väsyä forces the noun kesä into the illative case, any adjective attached to it must also take the exact same case.