Здавствуйте, всем, у меня вопрос. I'm practicing the Russian grammatical case system, and one of the ways I've been doing it is by forming my own sentences, like the ones below for the word "письмо":

Grammatical Case Example Sentence Translation
Nominative Это письмо This is a letter
Accusative Я пишу письмо I'm writing a letter
Genitive Письма контент The letter's content
Prepositional Я пишу на письме I'm writing on the letter
Dative Дай письму меня Give the letter to me
Instrumental Я отвечал через письмом I replied by letter

But my question is: are all of the above correct? If not, or if there's a better way of writing them, could you please correct me? Большое спасибо.