In the old times, it was common to put the subject after the predicate in a sentence that starts with an adverbial, an object, or a subclause. For example, the Bible translation from the 1930s starts with Alussa loi Jumala taivaan ja maan (In the beginning God created the heaven and the earth); the current one, from 1992, starts with Alussa Jumala loi taivaan ja maan.
The inverted order—predicate before subject—was an imitation of the grammar of Swedish and German. In the 20th century, it was declared as sveticistic (i.e., being influenced by the Swedish language) in language guides and in teaching. It became rare, but this purification of the language got partly overboard. Many people started to think that an inverted word order is always wrong, or almost always.
However, there are many situations where an inverted order is better or at least acceptable. It is normal to use inverted order when the subject refers to something that has not been mentioned before. For example, Tämän talon rakensi Jussi (This house was built by Jussi) would change its meaning if the order were different: Tämän talon Jussi rakensi would mean “This is the house that Jussi built”. The inverted word order Sodan takia joutuivat miljoonat ihmiset jättämään kotinsa (Due to the war, millions of people had to leave their homes) is also normal, even though it is nowadays often avoided and direct order is used due to the influence of English.
Inverted word order can also be used for reasons of rhythm and style. In particular, if the predicate is short, e.g. just on (is), and the subject is long, inverted order can make the sentence essentially easier to read and understand. The following example is from the language guide Uusi kieliopas: the direct order in the main clause in Jos lakiesitys hyväksytään, uusitun järjestelmän mukaiset korotetut taksat voidaan ottaa heti käyttöön makes the sentence more difficult to reader than the inverted order in Jos lakiesitys hyväksytään, voidaan uusitun järjestelmän mukaiset korotetut taksat ottaa heti käyttöön (If the bill will be approved, the increased fares of the renewed system can be taken into use immediately). When the subclause starts with the predicate voidaan, the structure of the subclause can be understood earlier.
When inverted word order is used and the predicate contains an auxiliary verb, only that verb is placed before the subject. Example: Tänään on korkein oikeus antanut asiasta tuomion (Today the Supreme Court has given a verdict on the matter). The predicate is the perfect tense form on antanut (has given), but only the auxiliary verb on is placed before the subject korkein oikeus.