Отопительный сезон уже начался.

Отопительный means heating in the sense of something related to heating.

It comes from the noun отопление (heating) or the verb отапливать / отопить (to heat a building). The suffix -тельный often makes adjectives meaning related to something.

So:

• отопление = heating
• отопительный = heating-related

That is why отопительный сезон means the heating season.

Сезон is in the nominative singular because it is the subject of the sentence.

The basic structure is:

• Отопительный сезон = the heating season
• уже = already
• начался = started / has started

So literally: The heating season has already started.

Russian adjectives must agree with the noun they describe in:

• gender
• number
• case

Here, сезон is:

• masculine
• singular
• nominative

So the adjective also has to be:

• masculine
• singular
• nominative

That gives отопительный сезон.

If the noun were different, the adjective would change too.

Because the verb here is начаться, not начать.

Compare:

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

So:

• Он начал урок. = He started the lesson.
• Урок начался. = The lesson started.

In your sentence, the subject is отопительный сезон, and the idea is the heating season started, so Russian uses начаться:

• Отопительный сезон уже начался.

The -ся makes it the intransitive form: the season itself began.

The ending -ся often marks a reflexive or intransitive verb. In this sentence, it does not mean the season literally started itself in a physical sense. Rather, it is the normal Russian way to say that something began.

So:

• начать = to start something
• начаться = to start, to begin

This is a very common pattern in Russian:

• открыть = to open something

• открыться = to open

• закончить = to finish something
• закончиться = to finish, come to an end

So начался is just the correct past-tense form of начаться for a masculine singular subject.

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

The noun сезон is masculine singular, so the verb must also be masculine singular:

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

Examples:

• Зима началась. = Winter started.
  (зима is feminine)

• Лето началось. = Summer started.
  (лето is neuter)

• Каникулы начались. = The holidays started.
  (каникулы is plural)

Уже means already, and it usually goes before the word or phrase it modifies. In this sentence, it modifies the verb idea has started:

• Отопительный сезон уже начался.

This is the most natural word order.

Russian word order is fairly flexible, so you may also see:

• Уже начался отопительный сезон.
• Отопительный сезон начался уже. (less neutral in tone)

But the version in your sentence is the standard, neutral way to say it.

It can correspond to either started or has started in English, depending on context.

Russian does not have a separate present perfect form like English has started. Instead, the simple past form often covers both ideas.

So:

• Отопительный сезон уже начался.

can be understood as:

• The heating season already started
• The heating season has already started

In natural English, has already started is usually the best translation here.

Yes, it is a very common set phrase, especially in Russian-speaking countries where central heating in buildings is turned on seasonally.

It refers to the period when residential heating is officially provided. So this is not just any poetic or abstract season; it often has an administrative, practical meaning.

You may also hear related expressions like:

• начало отопительного сезона = the start of the heating season
• открытие отопительного сезона = the opening/start of the heating season
• отопление включили = they turned the heating on

A rough pronunciation guide is:

ata-PEE-tyel-ny sye-ZON oo-ZHE na-chal-SYA

A few notes:

• о often sounds closer to a when unstressed.
• е in сезон is pronounced like ye after с.
• -лся in начался can be tricky; say it smoothly as part of the word.
• The stress is:
  • отопи́тельный
  • сезо́н
  • уже́
  • начался́

Yes. Russian has no articles like a or the.

So сезон by itself can mean:

• a season
• the season

The exact meaning depends on context.

In this sentence, it is naturally understood as the heating season, because the speaker means a specific heating season relevant to the situation.