Learn Finnish: Perhe osti yhdet rattaat vauvalle. - The family bought one stroller for the baby.

The word for a stroller, rattaat, is a plural-only noun, much like 'scissors' or 'pants' in English. When you count plural-only nouns, the number must agree and also be in the plural form. Therefore, you use yhdet (the plural of yksi) to say 'one stroller'.

Since yhdet rattaat is the object being bought, why isn't it in the partitive case?

Buying the stroller is a completed action, which means the sentence requires a total object. For plural words, a total object takes the nominative plural form. If the action were incomplete or ongoing, you would use the partitive plural instead (yksiä rattaita).

If the family bought two strollers, would the phrase become kaksi rattaita?

No, you would use the plural form of the number two, which is kahdet, and keep the noun in the nominative plural: kahdet rattaat. When counting plural-only nouns, the numeral always takes a plural form (yhdet, kahdet, kolmet) and the noun stays in the nominative plural. This completely bypasses the standard rule where numbers normally require a partitive singular noun.

What does the word rattaat literally mean?

The singular word ratas means a wheel or a cog, so the plural rattaat literally translates to 'wheels' or 'a wheeled cart'. In everyday modern Finnish, it acts as the default word for a baby stroller or buggy.