Learn Russian: Мы готовили крем весь день, а сделали торт за час. - We prepared the cream all day, but made the cake in an hour.

To express the duration of an ongoing action (how long something lasted), Russian uses the bare Accusative case without any preposition. This pairs with the imperfective verb готовили to show that making the cream was a continuous process that took the whole day.

Why do we use the preposition "за" in "за час"?

The preposition за followed by the Accusative case is used to show how long it took to completely finish an action (time to completion). It pairs perfectly with the perfective verb сделали to mean the cake was completely done "in an hour."

Why use "а" for "but" in this sentence instead of "но"?

The word а is a contrastive conjunction. It is used here because we are directly comparing and contrasting two different actions and their timeframes: the long process of the cream versus the quick completion of the cake. The word но is usually used for an unexpected obstacle or contradiction.

Why do "крем" and "торт" look like they are in the Nominative (dictionary) case?

They are actually in the Accusative case because they are the direct objects of the verbs (the things being prepared and made). However, for masculine inanimate nouns like крем and торт, the Accusative form looks exactly the same as the Nominative form.