Zajęcia zaczynają się dzisiaj.

In this sentence, zajęcia means classes, lessons, or course sessions, and Polish normally uses this word in the plural for that meaning.

Because zajęcia is the subject, the verb has to agree with it:

• zajęcia zaczynają się = the classes start / are starting

That is why you get zaczynają and not zaczyna.

A singular form zajęcie does exist, but it usually means something like an activity, an occupation, or a different kind of task/engagement, not the normal everyday word for class/classes in this context.

Yes. This is very common.

Polish often uses zajęcia where English might naturally say either class or classes, depending on context.

For example:

• Mam dziś zajęcia z polskiego. = I have Polish class today / I have Polish classes today

So even though the Polish word is grammatically plural, the most natural English translation is not always plural.

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

This is an important difference:

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

Compare:

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

So in Zajęcia zaczynają się dzisiaj, the classes themselves are beginning, which is why się is needed.

No, not in normal Polish.

Without się, zaczynać usually needs an object, because it means to start something. So Zajęcia zaczynają dzisiaj sounds incomplete or incorrect in this meaning.

The natural form is:

• Zajęcia zaczynają się dzisiaj.

Polish often uses the present tense for scheduled, planned, or near-future events, just like English sometimes does:

• The semester starts today.
• The train leaves at six.

So Zajęcia zaczynają się dzisiaj is completely natural for The classes start today.

It can also sound a bit like Classes are starting today, depending on context.

Yes. That is also correct.

The difference is mainly aspect and nuance:

• zaczynają się dzisiaj uses the imperfective verb and often sounds like a scheduled fact or a general statement about what happens today
• zaczną się dzisiaj uses the perfective future and points more directly to the future starting point

Both can work, and in many everyday situations the difference is small.

Because Polish does not have articles like a and the.

So Zajęcia zaczynają się dzisiaj could mean:

• Classes start today
• The classes start today

Context tells you which one is meant.

This is something English speakers often have to get used to: Polish leaves article-type information unstated unless some other word makes it clear.

Zajęcia is in the nominative plural, because it is the subject of the sentence — it is the thing that is starting.

Dzisiaj is an adverb meaning today. It does not change for case.

So the structure is basically:

• zajęcia = subject
• zaczynają się = verb
• dzisiaj = time adverb

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

The version you have:

• Zajęcia zaczynają się dzisiaj.

is a neutral, natural sentence.

You could also say:

• Dzisiaj zaczynają się zajęcia.

That version puts more focus on today.

So the words can move, but the emphasis changes. The original sentence is a very normal default order.

Yes. Both mean today.

So you can say:

• Zajęcia zaczynają się dzisiaj.
• Zajęcia zaczynają się dziś.

Both are natural. Dziś is simply shorter. In everyday Polish, both forms are common.