Learn Hardcore Finnish: Kesällä puisto on vihreä ja rauhallinen. - In the summer, the park is green and peaceful.

Kesällä is the adessive case of kesä (summer). In Finnish the adessive (ending -llä) often expresses “during” or “in” when talking about time. So kesällä literally means “on/at summer,” i.e. “during summer.”

What grammatical case is puisto in, and why doesn’t it change?

Puisto is in the nominative singular case. As the subject of the sentence, it stays in nominative. Only objects or certain adverbials would take other cases.

Why are the adjectives vihreä and rauhallinen in these forms?

Since they are predicate adjectives describing the singular noun puisto, they appear in nominative singular:

vihreä = green
rauhallinen = peaceful/calm
In Finnish, adjectives linked with olla (to be) don’t inflect for case or number beyond matching the subject.

Why is there no article like the before puisto?

Finnish has no definite or indefinite articles. Nouns stand alone and context tells you if something is “a” or “the.” Here puisto simply means “(the/a) park” depending on what the speaker has in mind.

Why is the verb on used here?

On is the present tense, third-person singular form of olla (“to be”). It corresponds to English “is.”
Examples:

Minä olen = I am
Sinä olet = You are
Hän on = He/She is

Could you reorder the sentence? For example, can you say Puisto on kesällä vihreä ja rauhallinen?

Yes. Finnish word order is quite flexible. Putting kesällä later shifts the emphasis slightly:

Kesällä puisto on… emphasizes the time (“In summer, the park is…”).
Puisto on kesällä… stresses the park itself, then adds when it’s green and calm.

How would you say this in the plural (“Parks are green and calm in summer”)?

You’d change both noun and adjectives to plural, and the verb to ovat: “Kesällä puistot ovat vihreät ja rauhalliset.”

puistot = parks (nominative plural)
ovat = are
vihreät, rauhalliset = plural adjectives