Here Finnish thinks very much like British English:

Use	Tense	Example
a finished time: yesterday, last year, when I was a child	imperfekti	Eilen söin pizzaa. — Yesterday I ate pizza.
something over and done	imperfekti	Asuin Bostonissa kaksikymmentä vuotta. — I lived in Boston for twenty years.
up to now, still true	perfekti	Olen asunut Helsingissä viisi kuukautta. — I've lived in Helsinki for five months.
experience, news, already / yet / ever	perfekti	Oletko koskaan syönyt poroa? — Have you ever eaten reindeer?

The test is the same as in English: if you can say when it happened, use the imperfekti. Milloin…? (when?) always takes it: Milloin tulit Suomeen? — When did you come to Finland?