Muszę się zdecydować, czy iść do kina, czy zostać w domu.

In Polish, the verb ending usually already shows the subject, so muszę by itself means I must / I have to.

• muszę = I must
• musisz = you must
• musi = he/she/it must

Because of that, ja is often omitted unless you want emphasis or contrast:

• Ja muszę się zdecydować, nie ty.
  I have to decide, not you.

In a neutral sentence, Muszę... sounds most natural.

Here się is part of the verb zdecydować się, which means to make up one’s mind / to decide.

So:

• zdecydować by itself can mean to decide something
• zdecydować się means to decide / make a decision oneself, decide on something

In this sentence, się is not easily translated word-for-word into English. It is just a normal part of this verb pattern.

Compare:

• Muszę się zdecydować.
  I have to make up my mind.
• Muszę zdecydować, co zrobić.
  I have to decide what to do.

Yes. In Polish, się is quite mobile, and you may hear:

• Muszę się zdecydować
• Muszę zdecydować się

Both are possible. In this sentence, Muszę się zdecydować sounds very natural and common.

Learners should know that się is often placed early in the sentence, but not usually at the very beginning.

This is mainly about aspect, which is very important in Polish verbs.

• decydować = imperfective
  focuses on the process, repeated action, or general idea of deciding
• zdecydować = perfective
  focuses on reaching a decision, the completed result

In Muszę się zdecydować, the speaker means I need to come to a decision, so the perfective verb zdecydować się is the natural choice.

Compare:

• Muszę się zdecydować do jutra.
  I need to make up my mind by tomorrow.
• Długo decyduję.
  I’m taking a long time deciding.

They are infinitives because they follow the idea of deciding whether to...

The structure is basically:

• Muszę się zdecydować, czy...
• I have to decide whether to...

So Polish uses infinitives just like English often does:

• czy iść do kina = whether to go to the cinema
• czy zostać w domu = whether to stay at home

This is a very common structure after verbs like decide, want, prefer, try, etc.

Polish often repeats czy in sentences that mean whether... or...

So:

• czy iść do kina, czy zostać w domu

literally works like:

• whether to go to the cinema, or whether to stay at home

This repetition is very natural in Polish. It helps clearly mark the alternatives.

You may also think of it as a standard pattern:

• Nie wiem, czy zostać, czy wyjść.
  I don’t know whether to stay or go out.
• Muszę zdecydować, czy kupić to, czy poczekać.
  I have to decide whether to buy it or wait.

Sometimes you may hear alternative phrasing, but in this kind of whether... or... construction, repeating czy is the most natural and standard choice.

So this sounds best:

• Muszę się zdecydować, czy iść do kina, czy zostać w domu.

Using albo would feel more like simply listing two options, while czy... czy... specifically matches the idea of uncertainty or a choice that has to be made.

Because iść do means to go to a place.

So:

• iść do kina = to go to the cinema
• iść do szkoły = to go to school
• iść do domu = to go home

After do, Polish uses the genitive case, which is why kino changes to kina.

Compare:

• Idę do kina.
  I’m going to the cinema.
• Jestem w kinie.
  I’m at the cinema.

So do kina shows movement toward the place, while w kinie would describe being inside it.

Because zostać w domu means to stay at home, so the idea is being/staying in a place, not moving to it.

• w domu = at home / in the house
• do domu = to home / homeward

Compare:

• Idę do domu.
  I’m going home.
• Zostaję w domu.
  I’m staying at home.

After w, Polish usually uses the locative case here, so dom becomes domu.

Because Polish nouns change form depending on their grammatical role in the sentence.

Here:

• do kina → kino changes to kina because do requires the genitive
• w domu → dom changes to domu because w here requires the locative

This is one of the core features of Polish grammar: prepositions often control the case of the noun after them.

Good question. They are related, but not identical.

• być w domu = to be at home
• zostać w domu = to stay at home

Zostać emphasizes the choice or result of remaining there instead of going somewhere else.

So in this sentence, zostać w domu fits well because the person is deciding between two options:

• go to the cinema
• stay at home

If you used być w domu, it would sound less natural here because the decision is about remaining at home.

The word order is quite natural and neutral.

• Muszę się zdecydować = main clause
• czy iść do kina, czy zostać w domu = the two options

Polish word order is more flexible than English, but that does not mean all orders sound equally natural. This version is a standard, smooth way to say it.

You could move some things around for emphasis, but the original sentence is probably the best default choice for learners.

Yes. A speaker might shorten or simplify it depending on context. For example:

• Muszę zdecydować, czy iść do kina, czy zostać w domu.
• Nie mogę się zdecydować, czy iść do kina, czy zostać w domu.
  I can’t decide whether to go to the cinema or stay home.

The original sentence is already completely natural. It sounds like standard, everyday Polish. The only thing that may vary in real speech is the placement of się or slight changes in style.

A rough pronunciation guide is:

MOO-sheh shyeh zdeh-tsee-DOH-vatch, chi yeeshch doh KEE-nah, chi zoh-STAHCH v DOH-moo

A few tricky sounds:

• sz in muszę sounds like sh
• się is roughly shyeh
• czy sounds like chi, but with a harder Polish cz sound
• iść has a difficult final cluster; learners often approximate it at first
• w domu is pronounced more like v domu

You do not need perfect pronunciation immediately, but się, czy, and iść are especially worth practicing.