Отправитель письма написал, что запрос нужно отправить до вечера.

Because письма is in the genitive singular and shows a relationship: the sender of the letter.

• отправитель = sender
• письмо = letter
• отправитель письма = sender of the letter

This is very common in Russian. Where English often uses of, Russian often uses the genitive case instead.

Not quite. Отправитель specifically means sender, the person who sent something.

So:

• отправитель письма = the sender of the letter
• автор письма = the author of the letter
• писатель = a writer, as in an author of books

In this sentence, отправитель письма focuses on the person who sent the letter, not necessarily on their identity as a writer.

Написал is the past tense, masculine singular, of написать.

This tells us that the subject, отправитель, is treated as:

• singular
• masculine grammatically

Russian past tense agrees with the subject in gender and number:

• написал = masculine
• написала = feminine
• написало = neuter
• написали = plural

Since отправитель is a masculine noun, написал is the expected form.

Что here means that and introduces a subordinate clause.

So the structure is:

• Отправитель письма написал = The sender of the letter wrote
• что запрос нужно отправить до вечера = that the request needs to be sent by evening

This works much like English that in reported statements.

Because запрос is the direct object of отправить, so it is in the accusative case.

For an inanimate masculine noun like запрос, the accusative form is the same as the nominative:

• nominative: запрос
• accusative: запрос

So although it looks unchanged, it is functioning as the object: to send the request.

Нужно means something like it is necessary, one must, or needs to.

Russian often uses this kind of impersonal construction without naming who has the obligation.

So:

• запрос нужно отправить = the request needs to be sent / it is necessary to send the request

If Russian wants to say who needs to do it, it can add a dative noun or pronoun:

• Нам нужно отправить запрос = We need to send the request
• Вам нужно отправить запрос = You need to send the request

In your sentence, the focus is on the necessity of the action, not on who must do it.

This is about aspect.

• отправить = perfective
• отправлять = imperfective

Here, the sentence is talking about sending the request as a single completed action that should happen before a deadline. That is why отправить is natural.

Compare:

• нужно отправить = it needs to be sent, as a completed act
• нужно отправлять = it is necessary to be sending / to send regularly / to send in general

With a deadline such as до вечера, Russian usually prefers the perfective because the result matters: the request must be sent by that time.

After до, Russian uses the genitive case.

So:

• вечер = evening
• до вечера = until evening / by evening / before evening

This is a fixed rule:

• до утра = until morning
• до понедельника = until Monday
• до конца недели = until the end of the week

So вечера is genitive singular.

It often means by evening or before evening comes, depending on context.

In this sentence, the natural meaning is that the request must be sent no later than the evening. In English, the best translation is often by evening.

Russian до marks a limit in time, so the action should happen before that point is reached.

No, Russian word order is fairly flexible, but the given order is neutral and natural.

Current order:

• Отправитель письма написал, что запрос нужно отправить до вечера.

This sounds like a normal, neutral statement.

Other orders are possible, for example:

• Отправитель письма написал, что до вечера нужно отправить запрос.

This puts a little more focus on the deadline до вечера.

Russian uses word order not only for grammar, but also for emphasis, focus, and flow.

Because что introduces a subordinate clause, and Russian normally separates subordinate clauses with a comma.

So the sentence is divided into:

• main clause: Отправитель письма написал
• subordinate clause: что запрос нужно отправить до вечера

This comma is required in standard Russian punctuation.

Yes, надо would also be possible in many contexts:

• запрос нужно отправить до вечера
• запрос надо отправить до вечера

Both mean roughly the request needs to be sent by evening.

Very broadly:

• нужно can sound slightly more neutral or formal
• надо can sound a bit more conversational

But in many real sentences, they are interchangeable.