Таможенное оформление начнётся завтра.

It is an adjective + noun phrase:

• таможенное = customs-related
• оформление = processing / оформление / formal registration / clearance, depending on context

Together, таможенное оформление means something like customs clearance or customs processing.

Both words are in the neuter singular nominative, because the whole phrase is the subject of the sentence.

Because it agrees with оформление, and оформление is a neuter noun.

You can often recognize neuter nouns by endings like:

• -о
• -е
• -ие

Here, оформление ends in -ие, so the adjective must also take a neuter singular form:

• masculine: таможенный
• feminine: таможенная
• neuter: таможенное

It is in the nominative case, because it is the thing that will begin.

In English, the subject is customs clearance. In Russian, the same role is played by таможенное оформление.

So the structure is basically:

• Таможенное оформление = the subject
• начнётся = will begin
• завтра = tomorrow

Because the verb here is начаться, not начать.

Compare:

• начать = to begin/start something
• начаться = to begin/start (of something beginning by itself)

So:

• Он начнёт работу. = He will start the work.
• Работа начнётся. = The work will begin.

In your sentence, таможенное оформление is what begins, so Russian uses the reflexive verb:

• начаться → начнётся

The -ся marks the verb as reflexive or, more broadly, as a verb that behaves differently from the non-reflexive version.

Here it does not literally mean it begins itself in a natural English sense. Instead, it is just part of the verb начаться, which means to begin.

This is very common in Russian:

• открыть = to open something

• открыться = to open / to become open

• начать = to start something
• начаться = to begin

So in this sentence, начнётся means will begin.

Russian often expresses future inside the verb form itself.

начнётся is the future tense form of начаться.

Russian has two main ways to form future:

1. Simple future with a perfective verb

   • начнётся = will begin

2. Compound future with быть

   • infinitive, usually with an imperfective verb
   • будет начинаться

In this sentence, the action is seen as a single event that will start, so Russian uses the perfective future:

• начнётся

Because начинается is the imperfective present form of начинаться.

That form can mean things like:

• is beginning
• begins
• sometimes a scheduled future in certain contexts

But for a clear, one-time future event like will begin tomorrow, Russian usually prefers the perfective future:

• начнётся завтра

So Таможенное оформление начнётся завтра sounds natural for Customs clearance will begin tomorrow.

It is pronounced approximately nach-NYOT-sa.

A few helpful points:

• the stress is on -ётся
• ё is pronounced yo
• тс in -ться / -тся is pronounced like ц in normal speech

So:

• начнётся → roughly nach-NYOT-sa

Also, in ordinary Russian writing, ё is often written as е, so you may see:

• начнется

But it is still pronounced начнётся.

The main word stresses are:

• тамо́женное
• оформле́ние
• начнётся
• за́втра

So the whole sentence is:

тамо́женное оформле́ние начнётся за́втра

Stress matters in Russian, so it is worth learning words together with their stress.

Because завтра works as an adverb meaning tomorrow.

Russian often uses time words directly, without a preposition:

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

So:

• начнётся завтра = will begin tomorrow

No extra word like on is needed.

Yes. Russian word order is relatively flexible.

The neutral order here is:

• Таможенное оформление начнётся завтра.

But you could also say:

• Завтра начнётся таможенное оформление.

This puts more emphasis on tomorrow.

In many cases, the meaning stays the same, but the focus or emphasis changes.

Yes, it sounds fairly formal and official.

That is because:

• таможенное оформление is administrative/business vocabulary
• the whole sentence sounds like something from logistics, shipping, travel, or customs documentation

In everyday speech, a person might still say it, but it especially fits official or professional contexts.

The dictionary forms are:

• таможенный = customs-related
• оформление = processing / registration / оформление
• начаться = to begin
• завтра = tomorrow

The verb form in the sentence is:

• начнётся = 3rd person singular future of начаться

That matches the singular subject таможенное оформление.