Замена начнётся завтра.

Замена is in the nominative singular. It is the subject of the sentence — the thing that will begin.

A quick breakdown:

• замена = nominative singular
• начнётся = will begin
• завтра = tomorrow

So grammatically, замена is the thing performing the action of beginning.

The infinitive is начаться.

This verb means to begin / to start in the sense that something begins.

The form начнётся is:

• 3rd person singular
• future tense
• from the perfective verb начаться

So the full pattern is:

• я начнусь
• ты начнёшься
• он / она / оно начнётся
• мы начнёмся
• вы начнётесь
• они начнутся

In this sentence, замена is singular, so the verb is начнётся.

Because начаться is a perfective verb.

In Russian:

• imperfective verbs can have a true present tense
• perfective verbs do not have a present tense form with present meaning

So the form начнётся may look like a present-style conjugation, but with a perfective verb it means future: will begin.

Compare:

• начинается = is beginning / begins (imperfective, present)
• начнётся = will begin (perfective, future)

Russian uses the perfective here because it focuses on the starting point as a single event.

The -ся makes the verb начаться, which is the intransitive counterpart of начать.

Compare:

• начать что-то = to start something
• начаться = to begin / to start (for something that begins itself, as the subject)

So:

• Они начали замену. = They started the replacement.
• Замена началась. = The replacement began.

In your sentence, замена is the thing that begins, so Russian uses начаться, not начать.

The stress is:

заме́на начнётся за́втра

A few useful pronunciation notes:

• ё in начнётся is always stressed
• -тся in speech sounds roughly like -ца
• so начнётся sounds approximately like nach-NYO-tsa

Very roughly in English-style transcription:

za-MYE-na nach-NYO-tsa ZAV-tra

If you are reading carefully as a learner, it is very helpful to remember the stress:

• заме́на
• начнётся
• за́втра

In standard spelling, the correct form is начнётся with ё.

However, in everyday Russian writing, native speakers often write е instead of ё, so you may also see:

Замена начнется завтра.

That is very common in real texts. But for learners, ё is extremely useful because it shows both:

• the correct vowel
• the stress

So when studying, it is better to remember the form as начнётся.

In this sentence, завтра functions as an adverb of time meaning tomorrow.

So the easiest way to understand it is simply:

• завтра = an adverb answering when?

That is why there is no preposition before it.

Russian often uses time words directly as adverbs:

• сегодня = today
• завтра = tomorrow
• вчера = yesterday

So Замена начнётся завтра is completely normal Russian word order and structure.

Yes. Russian word order is flexible.

The neutral version here is:

Замена начнётся завтра.

But you could also say:

• Завтра начнётся замена.
• Начнётся замена завтра.

The core meaning stays the same, but the emphasis changes:

• Замена начнётся завтра = neutral, matter-of-fact
• Завтра начнётся замена = stronger focus on tomorrow
• Начнётся замена завтра = possible, but less neutral in many contexts

So the original sentence is a very natural default order.

Russian does not have articles like the or a/an.

So замена can mean something like:

• a replacement
• the replacement

Which one is meant depends on the context.

Russian speakers use context, word order, and situation rather than articles to show whether something is definite or indefinite.

Not necessarily. Замена is a fairly broad noun.

Depending on context, it can refer to things like:

• replacement of a part or component
• substitution of one person for another
• a changeover
• a swap
• a roster or lineup replacement

So in a sentence like this, замена can describe an event or process that is scheduled to begin. The exact nuance depends on the wider context.

Grammatically, you could build a future expression with будет + imperfective, but it would usually sound less natural here.

Compare:

• Замена начнётся завтра. = the replacement will begin tomorrow
• Замена будет начинаться завтра. = the replacement will be starting tomorrow

The second version is more awkward in most ordinary contexts because Russian normally uses the perfective future when talking about the single event of something starting.

So начнётся is the natural choice here.