Learn Russian: Невозможно выучить язык за неделю. - It is impossible to learn a language in a week.

The perfective выучить focuses on the result or completion of the action—actually acquiring the language to fluency. If you used the imperfective учить, the sentence would mean "It is impossible to study a language in a week," which doesn't make logical sense, as you can easily study something for a week without mastering it.

Why is "in a week" translated as за неделю rather than в неделю or через неделю?

When expressing the amount of time it takes to complete an action from start to finish, Russian uses за followed by the accusative case. If you used через неделю, it would mean "a week from now" (as in, "I will learn it a week from now"). В неделю is used for frequency, such as "once a week" (раз в неделю).

Why does the sentence start with the adverb Невозможно instead of an adjective like Невозможный?

This is an impersonal construction. While English uses the dummy subject "It is...", Russian omits the pronoun entirely and uses a predicative adverb (often ending in -о) alongside an infinitive verb (выучить). You would only use the adjective невозможный if it were directly modifying a masculine noun, like невозможный план (an impossible plan).

What case is язык in, and why doesn't its ending change?