Не злись, пожалуйста, если я отвечу не сразу: сегодня я очень занята.

Не злись is the negative imperative of злиться — in other words, it means don’t get angry / don’t be mad.

A few useful points:

• злиться = to be angry, to get mad
• злись = singular informal command, used with ты
• не злись = don’t be mad

If you were speaking politely or to more than one person, you would say:

• Не злитесь

So this sentence is addressed to one person informally.

Yes. The base verb is злиться, which is a reflexive verb.

In Russian, many verbs simply exist in a reflexive form, and you often just have to learn them that way. Here, злиться means to get angry / to be angry, and the -ся / -сь is part of the normal dictionary form.

It does not necessarily mean a literal English-style oneself every time. So it is best to learn:

• злиться = to be mad / angry

not as a word-by-word combination, but as a single verb.

Отвечу is the perfective future of ответить.

Russian has two aspects:

• отвечать = imperfective
• ответить = perfective

In this sentence, the speaker means a single reply or the moment of replying, not an ongoing process. That is why Russian uses the perfective future:

• если я отвечу не сразу = if I don’t reply right away / if my reply is not immediate

Compare:

• отвечу = I will reply, one completed reply
• буду отвечать = I will be replying, process-focused
• отвечаю = I answer / I am answering, present tense

So отвечу fits best here.

That is a very common difference between English and Russian.

In English, after if, we usually use the present for future meaning:

• If I reply late...

But in Russian, after если, you can use the future when the condition is about the future:

• если я отвечу не сразу

So this is completely normal Russian grammar. Russian does not follow the same rule as English here.

This is an irregular-looking but normal verb pattern.

The infinitive is:

• ответить

But the 1st person singular future is:

• я отвечу

This kind of stem change happens with some Russian verbs, especially in certain -ить verbs.

For this verb, it is best to memorize the forms:

• я отвечу
• ты ответишь
• он/она ответит
• мы ответим
• вы ответите
• они ответят

So yes, отвечу may look surprising at first, but it is the correct form.

Не сразу means not immediately, not right away, or not at once.

Literally, сразу is something like:

• immediately
• at once
• right away

So:

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

This is a very common and natural expression in Russian.

Because занята is the short form of the adjective занятый and is used as part of the predicate:

• я занята = I am busy

Russian often uses short-form adjectives after the implied verb to be in the present tense.

Compare:

• занятая женщина = a busy woman
  full form, used before a noun
• я занята = I am busy
  short form, used as the predicate

So in this sentence, занята is exactly what Russian normally uses.

Because the speaker is female.

Russian adjectives in this kind of structure agree with the person’s gender:

• я занят = I am busy, said by a man
• я занята = I am busy, said by a woman

So this sentence tells you something about the speaker: she is female.

The colon introduces the reason or explanation for what came before.

The structure is basically:

• Don’t be mad if I don’t reply right away: today I’m very busy.

So the second part explains the first part.

You could think of it as:

• because today I’m very busy

Russian punctuation often uses a colon this way when the second clause explains or justifies the first.

Because пожалуйста here is a parenthetical politeness word, similar to please in English.

So it is set off with commas:

• Не злись, пожалуйста, если...

This is very common in Russian.

You will often see:

• Скажи, пожалуйста...
• Подождите, пожалуйста...
• Помоги, пожалуйста...

The commas show that пожалуйста is inserted into the sentence rather than functioning as a main sentence part.

The main change is the imperative:

• informal, to one person: Не злись
• formal or plural: Не злитесь

So the formal version would be:

• Не злитесь, пожалуйста, если я отвечу не сразу: сегодня я очень занята.

If the speaker were male, then the last word would change too:

• сегодня я очень занят