Alexander I was twenty-three years old when, following the deposition and assassination of his father, Emperor Paul, he ascended the Russian throne.
Александер I был двадцать восемч лет когда, следующий свержение и предательское из его, император пол, тот всходл русский троно.
Explaination: Since imperfective verb express a simple action in progress....Accusative because Russian throne is direct object..
Please explain mistakes in detail or correct the above..Thank you!