Partia zaczyna się jutro.

Not usually in the social event sense. A social party in Polish is more often impreza or przyjęcie.

Partia can mean different things depending on context, for example:

• a political party
• a game / round / match
• a batch / lot
• a part or set in some contexts

So partia is a context-dependent word, and it is a bit of a false friend for English speakers.

Here się is part of the verb expression zaczynać się / zacząć się, which means to begin or to start.

So:

• zaczynać = to start something
• zaczynać się = to begin / to start on its own

In this sentence, się does not mean itself in a literal English way. It is just how Polish forms this verb for events.

For example:

• Zaczynam lekcję. = I am starting the lesson.
• Lekcja zaczyna się. = The lesson is starting / begins.

With events, meetings, lessons, films, games, and similar things, Polish very often uses the reflexive form zaczynać się.

So:

• Partia zaczyna się jutro. = The game/match/batch begins tomorrow.

Without się, the verb more naturally means that someone starts something:

• On zaczyna partię. = He is starting the game.

So in your sentence, się shows that the event itself is beginning.

This is very normal in Polish. The present form of an imperfective verb can be used for a scheduled or expected future event, much like in English:

• The train leaves tomorrow.
• The game starts tomorrow.

So zaczyna się jutro is a natural way to say that something is due to begin tomorrow.

More technically, zaczyna is a non-past form, and here it refers to the future because of jutro.

Yes. That is also correct.

The difference is mainly aspect:

• zaczyna się jutro = imperfective, often used for scheduled or arranged future events
• zacznie się jutro = perfective future, meaning will begin tomorrow

In many real situations, both are possible. A rough comparison is:

• Partia zaczyna się jutro = The game starts tomorrow
• Partia zacznie się jutro = The game will start tomorrow

Zaczyna is the 3rd person singular non-past form of zaczynać się.

It matches the singular subject partia.

A useful thing to remember: in the present tense, Polish verbs do not show gender. So the same form works with masculine, feminine, and neuter singular subjects:

• Film zaczyna się jutro.
• Lekcja zaczyna się jutro.
• Spotkanie zaczyna się jutro.

All of these use zaczyna się.

Yes, partia is the subject of the sentence.

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

So the structure is:

• partia = subject
• zaczyna się = verb
• jutro = time expression

Here jutro is an adverb meaning tomorrow.

In this sentence, you do not treat it as a noun with a case ending. It stays the same:

• jutro = tomorrow

So learners usually just remember it as a time adverb.

Yes. Polish word order is more flexible than English word order.

The neutral version is:

• Partia zaczyna się jutro.

But you can also say:

• Jutro zaczyna się partia.

That version puts more focus on tomorrow.

So the meaning stays basically the same, but the emphasis can change.

Się usually stays close to the verb. In a neutral sentence, it often comes after the finite verb:

• Partia zaczyna się jutro.

That is the most natural basic pattern to learn first.

You may sometimes see other placements in real Polish, but for a learner, the safest rule is:

• keep się near the verb
• in simple neutral statements, putting it after the verb is usually best

Because Polish has no articles.

So partia can mean a game, the game, a batch, the batch, and so on, depending on context.

English needs articles, but Polish usually leaves that information to the situation or the wider sentence.