Learn Finnish: Ympäristö on likainen, koska sitä ei ole puhdistettu. - The environment is dirty because it has not been cleaned.

It is formed with the negative verb ei, the auxiliary verb ole (in its basic form without personal endings), and the passive past participle, which here is puhdistettu. We use the passive because the sentence doesn't say who failed to do the cleaning.

Why is the pronoun se (it) in the partitive form sitä?

In Finnish, the object of a negative sentence must always be in the partitive case. Because the action of cleaning did not happen (ei ole puhdistettu), the object receiving the action becomes sitä.

If the sentence were positive ('because it has been cleaned'), how would the verb phrase change?

The negative word ei would disappear, and the auxiliary verb ole would change to the third-person form on. The positive perfect passive is on puhdistettu.

How is the verb puhdistaa (to clean) related to the adjective for 'clean'?

The verb puhdistaa is built from the adjective puhdas (clean). The ending -staa is a common way to turn an adjective into a verb that means 'to make something [adjective]'.