Перед отправкой я быстро набрал текст на клавиатуре.

Because перед in the sense of before normally takes the instrumental case.

So:

• отправка → nominative
• перед отправкой → instrumental after перед

The same pattern appears in:

• перед уроком = before the lesson
• перед встречей = before the meeting
• перед отъездом = before departure

So перед отправкой means before sending or more literally before the sending.

No. Отправкой is a noun, from отправка.

Russian often uses nouns like this where English might prefer a verb. So instead of saying something like before I send it, Russian can say before sending / before the sending with a noun phrase:

• перед отправкой

This is very natural in Russian. A more explicitly verbal version would be:

• перед тем, как отправить

Both are possible, but the noun version is shorter and very common.

Yes, набрать can have several meanings depending on context, including:

• to collect / gather
• to dial
• to type in

In this sentence, because the object is текст and we also have на клавиатуре, the meaning is clearly typed in.

So набрать текст is a very common Russian expression meaning to type a text.

This is about aspect.

• набрал = perfective
• набирал = imperfective

Here набрал shows the action as completed: the speaker typed the text and finished doing so.

That fits well with the idea of doing it before sending. The sentence presents a single completed action:

• first, he typed the text
• then, it was ready to be sent

If you used набирал, it would sound more like the process was ongoing, repeated, or just being described without focus on completion.

All three are possible in some contexts, but they are not identical.

• написал текст = wrote the text
  This focuses on composing or writing it.
• напечатал текст = typed/printed the text
  This can work, but often emphasizes producing typed text, sometimes even printing depending on context.
• набрал текст = typed in the text
  This very naturally suggests entering the text on a keyboard.

So набрал текст на клавиатуре strongly points to the act of typing it in.

It is in the accusative.

The noun текст is:

• masculine
• singular
• inanimate

For masculine inanimate nouns, the accusative singular looks the same as the nominative singular.

So:

• nominative: текст
• accusative: текст

That is why the form does not change.

Compare:

• я вижу стол = I see the table
• я набрал текст = I typed the text

But with animate masculine nouns, the accusative would look different:

• я вижу брата

Yes, literally it does mean on the keyboard, but in Russian this is a normal way to express typing by means of a keyboard.

So набрать текст на клавиатуре means to type text on a keyboard / using a keyboard.

Russian often uses prepositional phrases in places where English might use a more purely instrumental wording.

A useful contrast:

• на клавиатуре = on the keyboard, using the keyboard
• с клавиатуры = from the keyboard, as input from the keyboard

For example:

• ввести данные с клавиатуры = to enter data from the keyboard

In your sentence, на клавиатуре is natural.

Yes, Russian often drops subject pronouns when they are clear from the verb.

So both are possible:

• Перед отправкой я быстро набрал текст на клавиатуре.
• Перед отправкой быстро набрал текст на клавиатуре.

Including я can do a few things:

• make the subject explicit
• add slight emphasis
• make the sentence clearer in isolation

So я is not required, but it is perfectly normal.

Russian word order is flexible, and adverbs like быстро can move around depending on emphasis.

Here:

• я быстро набрал текст = I quickly typed the text

This is a very natural neutral order.

You could also hear:

• я набрал текст быстро
• быстро я набрал текст на клавиатуре

But these can sound more marked or emphasize different parts of the sentence.

So the chosen order is simply a normal, natural way to say it.

In the past tense, Russian verbs agree in gender and number.

• набрал = masculine singular
• набрала = feminine singular
• набрало = neuter singular
• набрали = plural

So набрал tells you that the speaker or subject is grammatically masculine singular.

If a woman said the same sentence, it would be:

• Перед отправкой я быстро набрала текст на клавиатуре.

Yes. A common alternative is:

• Перед тем, как отправить, я быстро набрал текст на клавиатуре.

This is more explicitly verbal: before sending.

The difference is mainly stylistic:

• перед отправкой = more compact, noun-based
• перед тем, как отправить = more verbal, slightly more expanded

Both are natural. The original version is concise and very typical Russian.

Not exactly in form, even if the English translation may end up similar depending on context.

Перед отправкой literally means before sending or before the act of sending. It does not itself specify person, tense, or even explicitly who is doing the sending.

English often has to be more specific, but Russian can leave that understood from context.

So the phrase is structurally more general than before I sent it, even if the overall sentence makes the situation clear.