Learn Danish: Jeg køber en liter mælk. - I am buying a litre of milk.

In Danish, you don't use a preposition like af (of) between a unit of measurement and the item being measured. You just put the two nouns right next to each other, creating phrases like en liter mælk (literally 'a liter milk').

Why does the translation say 'I am buying' instead of 'I buy' when there is no '-ing' verb?

Danish doesn't have a present continuous tense like English does. The single present tense verb køber covers both 'I buy' as a general habit and 'I am buying' right now. You rely on the context to know which one makes sense.

How do I know that liter is an en word?

Liter is a common gender noun, which is why it uses en. Many everyday units of measurement take en, such as en flaske (a bottle) or en kop (a cup), but you still have to memorize them individually because there are exceptions like et kilo (a kilo).