Learn Finnish: Pese ikkuna heti, kello on jo kaksitoista! - Wash the window immediately, it is already twelve o'clock!

Because pese is an imperative (a command). When you command someone to do an action to a complete object, the object drops the normal -n ending and takes the nominative form. If the action was meant to be ongoing or incomplete, you would use the partitive ikkunaa.

Why is kaksitoista in the nominative here, rather than the partitive like in our current lesson?

Even though time feels abstract, telling the time is a fixed phrase based on a concrete object. The word kello literally means "clock", which is a concrete, countable noun. Because the subject is concrete, the number kaksitoista stays in the nominative case as the subject complement.

Does kello literally mean "o'clock" here?

It literally translates to "clock" or "watch". In Finnish, instead of saying "it is twelve o'clock" with a dummy pronoun, you literally say "the clock is twelve" (kello on kaksitoista).

What type of verb is pese and how is this command formed?

It comes from the type 3 verb pestä (to wash). To make the informal singular command form, you take the first-person singular form (pesen) and simply drop the personal -n ending to get pese.