Проверьте, пожалуйста, грамматику.

Russia is the largest country IN the world but AT the same time its population is smaller than in some other countries, China for example. Our country stretches FROM the Baltic sea TO the Pacific ocean.
There it borders on China and Japan. The chains OF mountains of the Pacific coast are not very high. The largest river of Russia is the Volga. It flows INTO the Caspian sea. A lot of old Russian cities lie iON it. The Volga rises FROM the Valday hills and flows TO the south. Russia is one of the greatest countries of the world and we are proud of IT.

Не знаю, что у вас должно быть...,но я исправляю вашу грамматику, не изменяя предложения... за лексику, равно как и за географию отвечаете вы!; -)
Russia is the largest country IN (OF) the world, but AT the same time its population is smaller than in some other countries, China for example. Our country stretches FROM the Baltic sea TO the Pacific ocean.
There it borders WITH China and Japan. The chains at THE mountains of the pacific coast are not very high. The largest river of Russia is the Volga. It flows INTO the Caspian sea. A lot of old Russian cities lie ALONG it. The Volga rises (???) in the Valday hills and flows from the south. Russia is one of the greatest countries of the world, and we are proud of IT.

Russia is the largest country IN the world, but AT the same time its population is smaller than IN some other countries, China FOR example. Our country stretches FROM the Baltic sea TO the pacific ocean.
There it borders ON (точно, проверила) China and Japan. The chains OF mountains OF (ON) the pacific coast are not very high. The largest river OF (IN) Russia is the Volga. It flows INTO the Caspian sea. A lot OF old Russian cities lie ON it. The Volga rises IN the Valday hills and flows TO the south. Russia is one OF the greatest countries of the world, and we are proud of it.