Если в письме есть срочное предупреждение, лучше не откладывать ответ до вечера.

Because в can take different cases depending on meaning:

• в + prepositional = location, meaning in / inside
• в + accusative = motion into something

Here the meaning is location: the warning is in the letter, so Russian uses в письме.

Compare:

• В письме есть предупреждение — There is a warning in the letter
• Положить записку в письмо — To put a note into the letter

Есть here means there is / there are in an existential sense.

So в письме есть срочное предупреждение literally means in the letter there is an urgent warning.

Russian often omits есть in the present tense when talking about identity or description, but it is commonly kept when you want to express existence or presence.

Compare:

• Это письмо важное — This letter is important
• В письме есть важная информация — There is important information in the letter

Without есть, the sentence can still be understandable, but есть sounds very natural here because the point is that such a warning is present in the letter.

Because предупреждение is a neuter noun, singular, and the adjective must agree with it.

So:

• предупреждение — neuter singular
• срочное — adjective in neuter singular form to match it

This is normal Russian agreement.

A few parallel examples:

• важное сообщение — an important message
• срочное письмо — an urgent letter
• срочная новость — urgent news item

The ending changes depending on the gender of the noun.

It is in the nominative case.

In this sentence, предупреждение is the thing that exists in the letter, so it functions like the subject of the clause with есть.

That is why you get:

• есть что? — срочное предупреждение

If the word had a different role in the sentence, the case might change, but here nominative is the expected form.

Because the sentence begins with a subordinate clause introduced by если — if.

Structure:

• Если в письме есть срочное предупреждение — subordinate clause
• лучше не откладывать ответ до вечера — main clause

Russian separates these with a comma.

If you reverse the order, the comma still stays:

• Лучше не откладывать ответ до вечера, если в письме есть срочное предупреждение.

Because лучше can be used impersonally, meaning something like it is better.

So:

• лучше не откладывать ответ = it is better not to postpone the reply

Russian often leaves the subject unstated in this kind of general advice. It sounds natural and idiomatic.

If you want to specify the person, you can:

• вам лучше не откладывать ответ — you had better not postpone the reply
• ему лучше не откладывать ответ — he had better not postpone the reply

But in your sentence, the advice is general, so no person is named.

Не откладывать is an infinitive, and after лучше it gives a general recommendation:

• Лучше не откладывать — It is better not to postpone

Не откладывайте is an imperative:

• Не откладывайте — Do not postpone

So the difference is tone:

• лучше не откладывать = neutral advice
• не откладывайте = direct instruction or command

Your sentence is phrased as advice, not as an order, so the infinitive is the natural choice.

Russian aspect matters a lot here.

• откладывать — imperfective
• отложить — perfective

With лучше не..., the imperfective is very natural when giving general advice about avoiding an action altogether. Не откладывать means do not put it off / do not engage in postponing it.

A perfective form here would sound much less natural, because it would focus on one completed act of postponing. The sentence is not really interested in that single completed result; it is giving broader advice: do not delay the reply.

So лучше не откладывать ответ is the idiomatic choice.

Because ответ is the direct object of откладывать, so it is in the accusative case.

For masculine inanimate nouns, the accusative singular is usually the same as the nominative singular. So:

• nominative: ответ
• accusative: ответ

That is why the form does not change.

Compare with a feminine noun, where the accusative would look different:

• откладывать встречу — postpone the meeting

But with ответ, the accusative happens to look identical to the dictionary form.

After the preposition до, Russian uses the genitive case.

So:

• до
  • genitive
• вечер → вечера

That is why the phrase is до вечера.

The meaning is until evening or before evening as the time limit or endpoint.

Other examples:

• до утра — until morning
• до понедельника — until Monday
• до конца дня — until the end of the day

Yes, but the meaning changes.

• до вечера = until evening, before evening, up to that point
• вечером = in the evening

So:

• не откладывать ответ до вечера = do not postpone the reply until evening
• ответить вечером = reply in the evening

Your sentence specifically warns against waiting that long, so до вечера is the right choice.

No, Russian word order is fairly flexible.

The given order is natural because it presents the condition first and then the advice:

• Если в письме есть срочное предупреждение, лучше не откладывать ответ до вечера.

But you could also say:

• Лучше не откладывать ответ до вечера, если в письме есть срочное предупреждение.

Both are grammatical. The difference is mainly one of emphasis and information flow.

The original version sounds very natural because it first sets up the condition, then gives the recommendation.

It can mean both, depending on context.

In modern Russian, письмо is often used for:

• a traditional letter
• an email
• sometimes a formal written message

So в письме can very naturally mean in the email as well as in the letter.

If the context is clearly digital, Russian speakers still often say в письме.