Use only one search key at a time. You can search for the postal code of a certain street or the street names and numbers falling under a certain postal code.
You will also find out the name of the municipality in which the post office in question is located or the postal codes that appear within the area of a certain municipality.
When searching for a street name, you may also use partial match search which requires at least 6 characters + %, for example by entering Kirkko% the search result will display all street names in Finland starting with the word 'Kirkko'.

A couple of words you may find useful:

kadunnimi = street name
postinumero = postal code
postitoimipaikka = post office
kunta = municipality
hae = search
tyhjennä = clear