Межевание началось сегодня.

Межевание is a noun, not a finite verb. It means something like land surveying / boundary demarcation / the process of marking property boundaries.

Grammatically, it is:

• neuter
• singular
• nominative case here, because it is the subject of the sentence

It is a common Russian pattern to use a noun ending in -ие to name a process or activity, especially in formal or official language.

So the sentence is structured like:

• межевание = the subject
• началось = began / started
• сегодня = today

Because the subject межевание is neuter singular.

In the past tense, Russian verbs agree with the subject in gender and number:

• masculine: начался
• feminine: началась
• neuter: началось
• plural: начались

Since межевание is neuter, the correct form is началось.

The verb here is начаться, not начать.

• начать = to start something
• начаться = to begin / to get started

So:

• Они начали работу. = They started the work.
• Работа началась. = The work began.

In your sentence, межевание is the thing that began, so Russian uses the reflexive verb начаться.

In the past neuter singular, that becomes началось. The final -сь is the reflexive ending.

This is an aspect question.

• началось is perfective
• начиналось is imperfective

Here, началось is used because the sentence reports a completed event of beginning: the process started today.

That is the normal choice when you are talking about the fact that something began.

By contrast, начиналось would usually sound more like:

• background description
• repeated or habitual starting
• focus on the process rather than the completed fact

So for a simple statement like It started today, началось is the natural form.

Yes, the word order can change.

Russian word order is much more flexible than English word order because the grammar is shown by endings, not mainly by position.

Межевание началось сегодня is a normal, neutral sentence.

You could also say:

• Сегодня началось межевание.
• Сегодня межевание началось.
• Межевание сегодня началось.

These all mean roughly the same thing, but the emphasis shifts a little.

A rough guide:

• Сегодня началось межевание. → emphasizes today
• Межевание началось сегодня. → neutral, or slightly leaves today as the new information
• Межевание сегодня началось. → possible, but a bit more marked in rhythm/emphasis

No. Сегодня is an adverb here, so it does not change for case.

It simply means today and answers the question when?

That is why it stays the same:

• Сегодня началось межевание.
• Сегодня холодно.
• Я работаю сегодня.

Unlike nouns, this word does not take case endings in this use.

Because Russian does not have articles like English the and a/an.

So Russian simply says:

• Межевание началось сегодня.

Depending on context, that can correspond to English:

• The boundary demarcation started today
• A boundary demarcation process started today
• The surveying began today

Russian leaves that kind of definiteness to context.

It sounds fairly formal or administrative, mainly because of the noun межевание.

This is the kind of word you might see in:

• legal documents
• land records
• municipal announcements
• official news or notices

In everyday conversation, people might choose a more descriptive wording depending on what exactly they mean. But the sentence itself is perfectly natural, especially in official contexts.

A helpful pronunciation guide is:

• межевание — roughly me-zhe-VA-ni-ye
• началось — roughly na-cha-LOS
• сегодня — roughly si-VOD-nya

Stress:

• межевАние
• началОсь
• сегОдня

One small note: in normal speech, сегодня is often pronounced closer to сиводня than the spelling might suggest.