В случае если срок действия истёк, начнётся замена.

В случае если is a more formal way to say if or in the event that.

• если = if
• в случае если = in the event that / in case

So this sentence sounds more official, bureaucratic, or written than a simple если срок действия истёк, ...

In everyday speech, Russians would often just say если.

Because the preposition в here is used in the expression в случае and requires the prepositional case.

The noun is:

• nominative: случай
• prepositional: случае

So:

• в случае = in the case / in the event

This is a fixed expression you will see often in formal Russian.

Срок действия is a very common formal phrase meaning period of validity, validity period, or sometimes expiration period/date, depending on context.

It is made of:

• срок = period, term, deadline
• действия = of validity / of operation / of functioning

Literally, it is something like the period of functioning/validity.

You will often see it with documents, cards, contracts, subscriptions, medicines, and so on.

Because действия is in the genitive singular and depends on срок.

This is a very common Russian noun pattern:

• срок чего? = the period of what?
• срок действия = the period of validity

So действия is not the subject here. It is part of a noun phrase describing срок.

Because the subject is срок, and срок is a masculine noun.

Past tense verbs in Russian agree in gender and number:

• masculine: истёк
• feminine: истекла
• neuter: истекло
• plural: истекли

So:

• срок истёк = the term expired
• виза истекла = the visa expired

Истёк is the past tense of истечь, which is a perfective verb.

Here that means the action is seen as completed:

• истекать = imperfective, to be expiring / to expire in a process sense
• истечь = perfective, to expire, to come to an end

So срок действия истёк means the validity period has already ended.

This is an important nuance.

• истёк = has expired / expired already
• истечёт = will expire

So в случае если срок действия истёк means:

if it turns out that the validity period has already expired

That is different from:

• в случае если срок действия истечёт = if the validity period expires / should expire in the future

The original sentence presents the expiration as something already completed by the time the condition is checked.

Начнётся is the 3rd person singular future of начаться, which is a perfective reflexive verb.

Breakdown:

• начаться = to begin, to start
• начнётся = it will begin

It agrees with замена, which is feminine singular, but in the future tense Russian verbs do not show gender, only person and number.

So the structure is:

• замена начнётся = the replacement will begin

Because the verb is начаться, not начать.

Compare:

• начать что-то = to begin something
• начаться = to begin, to start

In this sentence, замена itself is the thing that begins, so Russian uses the intransitive/reflexive form:

• начнётся замена = the replacement will begin

If you used начать, you would need an agent:

• компания начнёт замену = the company will begin the replacement

Замена is a noun meaning replacement.

In this sentence it is the grammatical subject of начнётся:

• замена начнётся = replacement will begin

This is a formal, somewhat impersonal way of speaking. Russian often does this in administrative or technical language, where the process matters more than who performs it.

The sentence does not say who will do the replacing or what exactly will be replaced, because that is either obvious from context or intentionally left unspecified.

Both are possible.

• начнётся замена sounds slightly more formal or process-focused
• замена начнётся sounds a bit more neutral and straightforward

Russian word order is flexible. Here, putting the verb first can make the result sound more official or procedural, which fits the tone of the whole sentence.

Yes.

The part в случае если срок действия истёк is a subordinate conditional clause, and начнётся замена is the main clause.

Russian normally separates these with a comma:

• В случае если срок действия истёк, начнётся замена.

So the comma is standard and expected.

Yes, in many texts you may see:

• истек
• начнется

instead of:

• истёк
• начнётся

In modern Russian, ё is often omitted in ordinary writing, although it is pronounced differently and is technically a separate letter.

So both spellings may appear in real texts, but the pronunciation is:

• истёк
• начнётся

For learners, it is helpful to remember the ё pronunciation even when it is not written.

Yes, very much so.

Several things make it sound formal or administrative:

• в случае если instead of simple если
• срок действия as an official set phrase
• начнётся замена as an impersonal procedural statement

This is the kind of Russian you might see in instructions, notices, contracts, technical documentation, or service messages.