Predicative cases

This chapter discusses predicative and whether to use the partitive or nominative case. This subject might be a little bit grammar heavy but I will try to make it as accessible as possible. It is however necessary to use some grammatical concepts, and we will begin with what the predicative is:

A typical predicative is a noun or an adjective which describes the subject of the sentence. Predicative and the subject are combined with the verb to be (olla). Here are some examples of a sentence with a predicative with the predicative underlined:
 * Minä olen mies / I am a man (nominative)
 * Kirja on kova / The book is hard (nominative)
 * Me olemme opettajia / We are teachers (partitive)
 * Ruoka on kylmää / The food is cold (partitive)

A subject which can be partitioned
The case partitive, as the name suggests, has something to do with parts. You should use the partitive when the subject can be partitioned. When is that?

The subject is material that can be partitioned If you have a plate of food and you split the food to plates, you still have food. If instead you have a sentence like "Mies on opettaja" (I am a teacher), you can't split a man in two and still have two men or even one. What is relevant is the semantical meaning of the subject. Let's have a couple of examples: A meal (ateria) is not really something to partition. Starters, main course and dessert is one meal and splitting it to two sets doesn't really make two meals. Both sentences about breakfast (aamupala) are correct. The difference in the meaning is that you the first might refer to the one specific plate you had infront of you while the latter might refer to all the optiones which were on the breakfast table and which you could share.
 * Ruoka on hyvää
 * Keitto on kylmää (The soup is cold)
 * Ateria on herkullinen / The meal is delicious
 * Aamupala oli herkullinen / The breakfast was delicious
 * Aamupala oli herkullista / The breakfast was delicious

The subject is plural

While a single man can't be partitioned and remain a man, a group of men can be split.

It is almost universally true that the predicative is in partitive when the subject is in plural. However, the rule is not grammatical but semantical. A pair of scissors is a good example:
 * Miehet ovat opettajia / The men are teachers
 * Appelsiinit ovat hyviä / The oranges are good
 * Kissat ovat pieniä / Cats are small
 * Sakset ovat punaiset / The pair scissors is red
 * Sakset ovat punaisia / (A group of) scissors are red

Even when you refer to just one item the word is in grammatical plural in both English and Finnish. You can split a scissor into two blades but that will not give you two scissors.

An advanced level example

Do you know this scene from Star Wars?

If there had been more droids on the loose, and not just two like at the back seat of the hover vehicle, you might have heard the following sentense
 * Nämä eivät ole etsimänne droidit / These aren't the droids you are looking for
 * Nämä eivät ole etsimiänne droideja / These aren't the (kind of) droids you are looking for

Predicative describing the material etc
3. On kylmää ja tuulista ja suomen kielioppikin on vaikeaa 4. Takki on nahkaa
 * It is cold and windy and the finnish grammar is difficult, too; some adjectives when they describe the state of things
 * Coat is made of leather; when describing a material