The word order is important when it comes to deciding whether a noun should be preceded by an indefinite or definite article. Simply put, the earlier a noun appears, the more likely it is that the noun is already known (and so the definite article is used).
- Tässä on tomaatti. - (Right) here is a tomato.
- Tomaatti on tässä. - The tomato is (right) here.
To be in order
There are no articles in Finnish. That does not mean that you can put any old article in the English translation of a Finnish sentence. Sentences with the verb olla, "to be", put nouns in a certain order. The more important and complete something is, the earlier it appears. If a noun ends the sentence, it is somehow incomplete, often because the word does not include everything it by definition could. This is why final nouns in sentences with the verb "to be" are translated with an indefinite article.
- Liisa on nainen.
- Liisa is a woman.
- Matti on mies.
- Matti is a man.
- Hän on velho.
- S/he is a wizard.
In sentences with the verb olla, "to be", the more complete a noun is, the earlier it appears. The later a noun appears, the less complete it is, and the more likely it is to be translated with an indefinite article.
- Hän on ujo poika.
- He is a shy boy.
However, the English language has so many ways of using articles in generalisations that sometimes an indefinite article starts such a sentence.
- Hyvä sauna on aina suomalainen.
- A good sauna is always Finnish.
In Finnish, that sauna is considered complete, since we are talking about all the good saunas in the world here.