Finnish has the same basic word order as English, the SVO order: subject, verb, object, for example Poikani sai kalan, “My son caught a fish”. It is the normal, neutral order that does not emphasize any of the words.
If another order is used, for some reason, the case suffixes usually distinguish between the subject and the object. For example, in the statement Matin voitti Pekka, the word Matin is in the genitive and Pekka is in the nominative, so Pekka is the subject and Matin is the object, i.e. the statement means “Matti was beaten by Pekka”. The use of this word order instead of the normal Pekka voitti Matin would be caused by desire to make Matti, who lost, the theme of the sentence.
However, case forms do not always indicate the subject and the object unambiguously. In particular, if both the subject and the object are in plural and a “total” object is used, they are in the same grammatical form. For example, the sentence Ässät voittivat Jokerit (Ässät beat Jokerit) is grammatically ambiguous, since both the subject and the object (which are plurals of nouns used as sports team names, “Aces” and “Jokers”) are in nominative plural. In such situations, the normal word order is used as a guideline to decide that the subject comes first.