Не сердись, пожалуйста, если я не отвечу сразу.

Because Не сердись is the imperative form, meaning don’t be angry / don’t get upset.

The dictionary form is сердиться = to be angry, to get angry.
To tell someone don’t be angry, Russian uses the imperative:

• сердись = be angry / get angry
• не сердись = don’t be angry

So сердиться is just the infinitive, while сердись is the form used for a command or request.

Because the verb is reflexive: сердиться.

Many Russian reflexive verbs end in -ся or -сь depending on the form. In the imperative singular, -ся often becomes -сь:

• учиться → учись
• беспокоиться → беспокойся
• сердиться → сердись

So сердись is simply the imperative form of the reflexive verb сердиться.

Yes. Не сердись is the singular informal imperative, used with ты.

If you are speaking politely to one person, or to several people, you would normally say:

• Не сердитесь, пожалуйста, если я не отвечу сразу.

So the sentence given sounds like it is addressed to a friend, family member, child, or someone you are on ты terms with.

Here сердиться means something like:

• to be angry
• to get upset
• to be annoyed

In this sentence, Не сердись is very natural as Don’t be upset / Don’t be mad.

It does not always mean strong anger. In everyday speech it can be fairly mild, like asking someone not to take offense or not to be annoyed.

Пожалуйста means please, and in Russian it is often inserted quite freely in a sentence.

So:

• Не сердись, пожалуйста = Please don’t be upset

Putting it after the imperative sounds very natural. Russian often places пожалуйста right after the request itself.

Because не отвечу is the future form: I won’t reply / I don’t reply right away in the specific future situation being mentioned.

The verb here is ответить, which is perfective. Its future is a simple one-word form:

• я отвечу = I will reply

So:

• если я не отвечу сразу = if I don’t reply right away

By contrast, не отвечаю is present tense and would mean I am not replying / I don’t reply, which does not fit this sentence as well. The speaker is talking about a possible future situation.

Russian often uses the perfective verb when talking about a single completed action in the future.

Here the idea is not if I am not in the process of answering, but if I do not send a reply right away. That is viewed as one completed action, so perfective ответить is natural:

• ответить → я отвечу

If you used the imperfective, it would suggest a different nuance, more about process or repeated behavior.

Сразу means:

• right away
• immediately
• at once

So если я не отвечу сразу means if I don’t reply right away / immediately.

It is a very common adverb in Russian.

There are actually two reasons for the commas in this sentence.

1. пожалуйста is treated as a parenthetical politeness word, so it is often set off by commas:

   • Не сердись, пожалуйста, ...

2. если introduces a subordinate clause:

   • ..., если я не отвечу сразу.

So the commas help separate both the polite insertion пожалуйста and the if clause.

Yes, Russian word order is flexible. For example, these are all possible:

• Не сердись, пожалуйста, если я не отвечу сразу.
• Пожалуйста, не сердись, если я не отвечу сразу.
• Если я не отвечу сразу, не сердись, пожалуйста.

They all mean basically the same thing, but the emphasis changes slightly.

The original version sounds very natural and conversational.

Yes. In this sentence, если simply introduces the condition:

• если я не отвечу сразу = if I don’t reply right away

This is one of the most basic and common uses of если.

Yes, you could, but the tone is a little different.

• не злись = don’t be angry
• не сердись = don’t be upset / don’t be mad

Both are possible, but не сердись often sounds a bit softer and very natural in this kind of sentence. It is especially common when you are asking someone not to take offense if you answer late.

Because Russian does not need to say at me in this sentence.

English often says Don’t be mad at me, but Russian can simply say:

• Не сердись = Don’t be upset / Don’t be angry

The object is understood from context. If needed, Russian could make it explicit:

• Не сердись на меня = Don’t be angry with me

But in your sentence, leaving it out sounds natural and idiomatic.