Learn Finnish: Autiolla kadulla ei ole ketään. - There is nobody on the deserted street.

Ketään is the partitive form of kukaan. We use the partitive case because this is a negative existential sentence ("There is not..."). When you state that something does not exist in a certain place, the subject of that sentence must be in the partitive.

Why does the sentence start with the location autiolla kadulla instead of the subject?

This is an existential sentence, which expresses that something exists (or in this case, doesn't exist) somewhere. Finnish existential sentences typically follow a "Location + verb + subject" word order. Translating it literally, it says "On the deserted street is not anyone."

Why does the adjective autio get the -lla ending too?

In Finnish, adjectives must match the noun they describe in both case and number. Because the street is in the adessive case (kadulla), the word for deserted must also be in the adessive case, becoming autiolla.

What happened to the letter "t" from the dictionary form katu?

Adding the -lla ending closes the final syllable of the word. This triggers consonant gradation, causing the strong grade "t" in katu to soften into the weak grade "d" in kadulla.