Если я не успею договорить сейчас, я перезвоню позже.

Успею is the 1st person singular future form of успеть.

The reason it may look unusual is that успеть is a perfective verb. In Russian, perfective verbs do not have a true present tense. What looks like a present form is actually a simple future:

• я успею = I will manage / I will have time
• ты успеешь
• он/она успеет

So Если я не успею... means If I don’t manage... / If I don’t have time...

Also, unlike English, Russian normally uses the future after если when the meaning is future:

• Если я не успею... = If I don’t manage... not a present form like English if I don’t.

That’s a very common question. Russian and English behave differently here.

In English, future time in if-clauses is usually expressed with the present:

• If I don’t finish now, I’ll call back later.

In Russian, when the situation is really about the future, you usually use the future:

• Если я не успею договорить сейчас, я перезвоню позже.

So the pattern is:

• English: If + present, will + verb
• Russian: Если + future, future

This is completely normal in Russian and does not sound strange.

The pattern успеть + infinitive means to manage to do something in time or to have enough time to do something.

So:

• успеть договорить = to manage to finish speaking
• не успеть договорить = not to manage to finish speaking

In your sentence:

• Если я не успею договорить сейчас... = If I don’t manage to finish speaking now...

This construction is extremely common in Russian:

• Я не успел поесть. = I didn’t manage to eat.
• Ты успеешь прийти? = Will you manage to come in time?

Договорить means to finish saying something or to finish speaking.

It comes from говорить (to speak / say) with the prefix до-, which often adds the idea of reaching the end of an action.

So:

• говорить = to speak
• договорить = to finish speaking / finish what one was saying

In this sentence, договорить suggests:

• the speaker has already started speaking
• but may not be able to finish right now

That is why it fits so well with успеть:

• не успею договорить = I won’t manage to finish speaking

They are related, but not the same.

• сказать = to say / tell something, often as a single completed act
• договорить = to finish saying something that has already begun

Compare:

• Я хочу сказать кое-что.
  = I want to say something.

• Подожди, я не договорил.
  = Wait, I haven’t finished speaking.

So in your sentence, договорить is more natural than сказать because the idea is not just to say, but to finish what I’m in the middle of saying.

They look similar, but they mean very different things.

• договорить = to finish speaking
• договориться = to come to an agreement / make an arrangement

Examples:

• Я не договорил. = I didn’t finish speaking.
• Мы договорились встретиться завтра. = We agreed to meet tomorrow.

So in your sentence, договорить has nothing to do with agreement. It simply means finish talking.

Перезвоню means I’ll call back or I’ll call again.

That prefix пере- here gives the idea of a repeated call:

• звонить / позвонить = to call
• перезвонить = to call back

So:

• Я позвоню позже. = I’ll call later.
• Я перезвоню позже. = I’ll call back later.

In your sentence, перезвоню is especially natural because the speaker is already in a conversation and is saying that if they cannot finish now, they will call back later.

Because the sentence is about single, completed actions in the future.

1. успею

Perfective успеть focuses on whether the result is achieved:

• Will I manage in time or not?

2. договорить

Perfective договорить focuses on finishing the speech:

• to reach the end of what I’m saying

3. перезвоню

Perfective перезвонить refers to one completed callback:

• I will call back later

If you used imperfective verbs, the meaning would shift toward process, repetition, or general activity, and it would sound less natural here.

This sentence is about:

• one possible unfinished conversation now
• one callback later

So perfective verbs are the natural choice.

Yes, the second я can be omitted:

• Если я не успею договорить сейчас, перезвоню позже.

That is perfectly natural in Russian.

But repeating я is also very common, especially because it:

• makes the sentence clearer
• balances the two clauses
• adds a slight sense of emphasis or explicitness

So both versions are fine:

• Если я не успею договорить сейчас, я перезвоню позже.
• Если я не успею договорить сейчас, перезвоню позже.

The version with both я sounds a little more explicit and careful.

Because Russian separates the two clauses of a complex sentence with a comma:

• Если я не успею договорить сейчас, я перезвоню позже.

The first clause is the if-clause:

• Если я не успею договорить сейчас

The second clause is the main clause:

• я перезвоню позже

This comma is required in standard Russian spelling.

Yes, it is conditional in meaning, but it is a real future condition, not a hypothetical or contrary-to-fact one.

Without бы

This sentence means a real possibility:

• If I don’t manage to finish now, I’ll call back later.

That is why Russian uses ordinary future forms:

• не успею
• перезвоню

With бы

Бы is used for more hypothetical, unreal, or dependent situations, for example:

• Если бы у меня было время, я бы остался. = If I had time, I would stay.

So your sentence does not use бы because the speaker is talking about a realistic future possibility, not an unreal one.

They mark the time contrast very clearly:

• сейчас = now / right now
• позже = later

So the sentence sets up a contrast:

• if I cannot finish now
• I will call back later

That contrast is a big part of why the sentence sounds natural and complete.

You could move them for emphasis, for example:

• Если я сейчас не успею договорить, я перезвоню позже.
• Если я не успею договорить, я позже перезвоню.

But the original version is the most neutral and natural.

You can understand it that way, but it is not the most natural English translation.

A more literal breakdown is:

• Если = if
• я не успею = I don’t manage / I won’t have time
• договорить = to finish speaking
• сейчас = now
• я перезвоню позже = I’ll call back later

So the sentence is built around the idea of managing in time.

Natural English versions include:

• If I can’t finish talking now, I’ll call back later.
• If I don’t manage to finish now, I’ll call back later.
• If I don’t have time to finish now, I’ll call back later.

All of these capture the sense of не успею договорить well.