Learn Portuguese (Portugal): Eu compro um litro de leite.

Um means "one" or "a/an", and it has to match the gender of the noun it counts. Since litro (litre) is a masculine word, we use the masculine um. As you learn larger numbers, this rule still applies to numbers ending in one: you would say trinta e um litros (thirty-one litres) but trinta e uma garrafas (thirty-one bottles).

Is the word de used exactly like "of" in English here?

Yes! When expressing quantities or measurements, Portuguese uses de to connect the measurement to the substance, just like in English. So, um litro de leite translates perfectly to "a litre of milk".

How does the verb comprar (to buy) change to compro?

Comprar is a regular verb ending in "-ar". To say "I buy" (eu compro), you drop the "-ar" ending from the dictionary form and replace it with "-o".

Does the word um match with litro or leite?