Subject and object in sentences

Nominative, genitive and partitive are the most important cases. They have special grammatical functions, that's why they are often called grammatical cases. The most important function is to distinguish the subject and the object.

Nominative

Subject is often in nominative case. Here koira (a dog) is in nominative, thus it's the object. These 2 sentences have the same meaning despite the word order:

If mies (a man) is in nominative case, it will change the meaning.

Now the man is the subject. Because the word order is quite free in Finnish, the case tells us who bit who.

Sometimes the object can be in nominative.

The component after "olla" verb (to be) is called the predicative. Predicative should not be mixed with the object of the sentence. (see more about this sentence type). They describe the subject, and they are in nominative:

Talvi on kylmä.
Winter is cold.
Hanki on valkoinen.
The snowdrift is white.

Genitive (n-object)

Typically the subject is in nominative and object is either in partitive (see below) or genitive (some grammars also call it the "n-object").

Mies söi leivän.
A man ate the bread.

Partitive

Basic meaning of the partitive case is part of something, or undefinite amount.

Pihalla oli lunta.
There was (some) snow outside.
Vesi oli kylmää.
The water was cold.
Saisinko vettä?
May I have (some) water?

Like the genitive, also the partitive can mark the object:

Mies söi leipää.
A man ate (some) bread.

Thus: the subject is in nominative case and the object eather in genitive or partitive.

Genitive or partitive?

Object of the sentence can be in genitive or partitive case. The typical difference is, if the process is finished:

Another difference is between the positive and negative sentence:

Ostan tuon kirjan
I will buy that book
En osta tuota kirjaa.
I won't buy that book.

If the amount is indefinte, the noun is in partitive:

Ostan pullon viiniä.
I'll buy a bottle of wine.
Ostan kaupasta viiniä ja leipää.
I'll buy some wine and bread from the shop.