Partitive case: when Finnish says 'some'

English uses "the" or "a" and leaves the object unchanged. Finnish marks the object with a case, and the partitive (osanmuoto) is the case for an object that is only partly affected. Ask yourself whether the thing is whole and finished, or partial, unfinished, or negated. If it is partial, use the partitive.

The short version: partitive = some, part of, not finished, or not.

Use the partitive for

• Partial amounts and uncountable things: Juon kahvia. (I drink (some) coffee.) Compare Join kahvin. (I drink the coffee, all of it.)
• Numbers and quantities: Ostin kaksi kirjaa. (I bought two books.) The number leaves the object open, so it takes the partitive.
• Negatives: En näe taloa. (I can't see the house.) Any negated verb puts its object in the partitive.
• Unfinished actions: Luin kirjaa. (I read (part of) a book.) Compare Luin kirjan. (I read the whole book.)

Forms

Most singular partitives end in -a or -ä. Some stems change in the process, so learn the pattern for each noun:

The plural partitive usually ends in -ja or -jä: kirjoja (books), taloja (houses).

Common mistakes

• Using the nominative after a number. Wrong: Ostin kaksi kirja. Right: Ostin kaksi kirjaa.
• Forgetting the negative. Wrong: En näe talo. Right: En näe taloa.
• Treating the partitive as the "normal" object. Think of it as the marked form. The accusative (kirjan) is the complete-action form, and it is the one that needs a reason to be chosen.