Learn Russian: Они пекли торт час, а сделали крем за пять минут. - They baked the cake for an hour, but made the cream in five minutes.

This is the core of our lesson! We use the bare accusative case with no preposition (час) to show how long an ongoing process lasted. We use за + accusative (за пять минут) to show how long it took to completely finish an action.

How do the verbs пекли and сделали match these time expressions?

The imperfective verb пекли (were baking) focuses on the ongoing process, so it naturally pairs with the duration час. The perfective verb сделали (finished making) focuses on the successful result, so it pairs with the completion time за пять минут.

If за requires the accusative case, why is минут in the genitive plural?

The preposition за does take the accusative case, and the number пять is exactly that: accusative. However, in Russian, any noun following a number 5 or higher must be in the genitive plural form. That is why we say пять минут instead of using an accusative noun.

Why use the conjunction а instead of но for "but"?

We use а here because we are drawing a direct contrast or comparison between two things: a long baking process versus a quick cream-making process. The word но is used for unexpected contradictions, such as "They baked a cake, but forgot the sugar."