Learn Finnish: Missä iso katu on? - Where is the big street?

In Finnish questions starting with a question word like missä, the verb often comes at the end if it is a simple location question. So instead of 'Where is the big street?', Finnish logically orders it as 'Where the big street is?' (Missä iso katu on?).

Is the question word missä related to mikä (what)?

Yes! The ending -ssä means 'in', so missä literally means 'in what'. This is how Finnish forms the word for 'where'.

How do we know this means 'the' big street and not 'a' big street?

Finnish does not have articles like 'a' or 'the'. You have to figure it out from context. Iso katu can mean 'a big street' or 'the big street', but since you are asking where it is, you are almost always looking for a specific one.

Why doesn't the adjective iso have any special endings here?

Adjectives must match the noun they describe. Because the noun katu (street) is the subject of the sentence and is in its basic dictionary form, the adjective iso (big) also stays in its basic dictionary form.