Questions & Answers about Kaupunki ostaa uusia kuorma-autoja.
Why are uusia and kuorma-autoja in the partitive plural instead of the regular basic plural?
In Finnish, the object of a sentence is in the partitive plural when we are talking about an unspecified amount of something, like 'some new trucks'. If the sentence used the basic plural (Kaupunki ostaa uudet kuorma-autot), it would mean the city is buying a specific, known set of trucks, translating closer to 'The city is buying the new trucks'.
How can I tell that uusia is plural? Doesn't it just look like the basic word uusi with an -a at the end?
It does look like that, but remember that the singular partitive of uusi is actually uutta. The plural stem for this word happens to be uusi-. When you add the partitive ending -a to that plural stem, you get the plural partitive form uusia.
Why is uusia in the partitive plural too, rather than just the noun?
Adjectives must always match the noun they describe in both case (here, partitive) and number (here, plural). Since the noun kuorma-autoja is in the partitive plural, the adjective modifying it must take the exact same form.
Why doesn't the kuorma part of kuorma-autoja change to plural?
Kuorma-auto (truck, literally 'load-car') is a compound word made of kuorma and auto. In Finnish, only the final word in a compound noun takes the grammatical endings. The first part stays in its basic dictionary form.