Questions & Answers about Мы купили две тонны картофеля.
Why do we use две here instead of два?
The number two has different forms depending on gender. Because тонна (ton) is a feminine noun, we must use the feminine form две.
Why is тонны in the genitive singular? Shouldn't it be plural if there are two of them?
In Russian, when a noun follows the numbers 2, 3, or 4 (in the nominative or inanimate accusative case), it must take the genitive singular form. So, the feminine noun тонна becomes тонны.
In English we say "potatoes" in the plural. Why is картофеля singular here?
The word картофель is a collective noun in Russian, behaving like mass nouns such as "rice" or "water" in English. It refers to the crop as a whole rather than individual potatoes, so it stays singular. It takes the genitive case here to express "of potatoes".
How would I say "one and a half tons of potatoes" using the grammar from this lesson?
You would say Мы купили полторы тонны картофеля. Because тонна is feminine, you use the feminine fraction полторы instead of the masculine/neuter полтора. Just like the number two, полторы requires the noun that follows it to be in the genitive singular.