Learn Finnish: Täällä hiekka ei ole puhdasta.

Because the subject of the sentence, hiekka (sand), is an uncountable mass noun. In Finnish, when an adjective describes a mass noun like this, the adjective must be in the partitive case.

Why is hiekka in the nominative case if it is a mass noun?

Hiekka is the subject of the sentence, and subjects are usually in the basic nominative case. The partitive rule for mass nouns in this lesson only applies to the adjective describing the subject (puhdasta), not the subject itself.

How do we form the partitive puhdasta from the basic word puhdas?

Adjectives ending in -as or -äs follow a very straightforward rule for the partitive case. You simply add the ending -ta or -tä directly to the basic form, turning puhdas into puhdasta.

Why does the sentence start with Täällä instead of the subject Hiekka?

Finnish word order is flexible and often places the most important contextual information at the beginning. Starting with Täällä emphasizes the location, similar to saying "Here, the sand is not clean" in English.