Падеж: Письмо

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.

The dative sentence is wrong. That письму is 'to (a) letter", and меня is me in accusative, so that sentence is "Give me to the letter". "Give the letter to me, me is dative, letter is accusative. Письму is the dative form though, I believe.
I'm pretty sure через matches with accusative, not instrumental (Which in most cases, means it will look like it matches with nominative). "I replied by letter" would (non native here) be more like Я ответил по письму.(that being dative)
For instrumental, you'd need words like под, or над. Exposure to russian writing will sort everything out over time. Eventually you see the word над enough, until you know which case it matches with (which varies depending on the situation). In a large number of situations, if a preposition is used with a verb (Which is arguably rare with russian), нa + prep is the way to go. Because the key of the whole system, is that there are no "floatin word". No word is just... There. Every word has it's reason appointed, and is connected logically to something else. Keep in mind, various propositions have different meanings when used with different case. Mainly, с(о). It could mean "with" (+inst), or mean "from" (+ gen), when talking about motion from somewhere, as opposed to "from" as in "origin". Motion generally changes the case. Под + inst= under .... Под + acc= to under, moving under.

Also, just keep in mind, everyone will tell you there are 3 genders, about three declensions, and six cases. Which is certainly enough to get by understanding. But also note that words like время and имя are not the same as лексика in any way, just because they all end in an a sound. Generally, prepositional covers the whole deal, but in some words, their system is split into Nom, Acc, Gen, Dat, Inst, Prepositional, and locative. As well as certain words also can be found in a "partitive" case (which remarkably resembles genitive in many ways, but is a little different). Exposure teaches everything over time.

Genitive: The letter's content
Письма контент
1) It’s correct but sounds unusual for everyday speech. Usually "object" is first, and "owner" is second: so it should be "Контент письма".
2) The "контент" is loan word and it’s used in technical text. Use the word "содержание" instead "контент" in everyday speech.
Should be: Содержание письма.


Dative: Give the letter to me
Дай письму меня
Should be: Дай письмо мне.


Instrumental: I replied by letter
Я отвечал через письмом.
1) "Через" means "through", not "by".
2) Replied (past simple), so you should use perfective aspect.
Should be: Я ответил письмом
or... Я ответил посредством (при помощи) письма... However another case is used in this constructions

Quote:

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Originally Posted by Звездочёт

1) "Через" means "throw", not "by".
Should be: Я ответил письмом

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I don't like to be nit-picky about typos (considering what a nightmare English spelling is), but since Missionary is a beginner, I want to point out that Звездочёт meant to write "through" (через), not "throw" (бросать).

P.S. Звездочёт, does your screen-name mean "he who counts stars"? I'm reminded of пулемёт ("machine-gun," literally "bullet-thrower") and also the notorious маленький зелёный камнеед ("little green rock-eater").

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I want to point out that Звездочёт meant to write "through" (через), not "throw" (бросать).

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Yes. I also found this mistake. Corrected.

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P.S. Звездочёт, does your screen-name mean "he who counts stars"?

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Yes :)

Спасибо, всем! :)