Learn Spanish (Spain): Compro un litro de leche.

In Spanish, the ending of the verb usually tells you who is doing the action. Because compro ends in -o, it already means "I buy." You can add yo (yo compro) for emphasis, but it is usually left out.

Why do we say de leche instead of just litro leche?

Just like in English where we say "a liter OF milk," Spanish uses de (of) to connect units of measurement to the item being measured. You will use this same pattern for other measurements, like un kilo de manzanas (a kilo of apples).

Why isn't it de la leche since milk is a specific noun?

When talking about a general substance after a measurement, you do not use the article (la or el). You just use de directly followed by the noun, making it un litro de leche.

Since we don't see el or la here, how do I know if leche is masculine or feminine?

You cannot tell just from this sentence! However, leche is a feminine word (la leche). Even though it ends in -e instead of the typical feminine -a, it is a very common exception that you will learn quickly.