W sobotę muszę wyczyścić akwarium.

Because after w when talking about time in expressions like on Saturday, Polish uses the accusative case.

• sobota = nominative dictionary form
• w sobotę = accusative, meaning on Saturday

This is a very common pattern:

• w poniedziałek = on Monday
• we wtorek = on Tuesday
• w środę = on Wednesday
• w czwartek = on Thursday
• w piątek = on Friday
• w sobotę = on Saturday
• w niedzielę = on Sunday

Notice that not all weekdays change in the same way, so it is best to learn them as whole expressions.

Muszę means I must or I have to.

Its base form is musieć = to have to / must.

This is the 1st person singular form, so:

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

In this sentence, muszę is followed by an infinitive:

• muszę wyczyścić = I have to clean

That is a very common structure in Polish:

• muszę pracować = I have to work
• muszę zadzwonić = I have to call

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

• czyścić = to be cleaning / to clean in an ongoing, repeated, or general sense
  → imperfective
• wyczyścić = to clean completely / finish cleaning
  → perfective

In W sobotę muszę wyczyścić akwarium, the speaker means a completed task: the aquarium needs to get cleaned, not just be in the process of cleaning.

So:

• muszę czyścić akwarium can suggest I have to be cleaning / I need to clean it regularly / I need to spend time cleaning it
• muszę wyczyścić akwarium means I need to get it cleaned completely

With muszę, both imperfective and perfective are possible, but they give slightly different meanings.

The prefix wy- often adds the idea of doing something thoroughly, out, or to completion.

So:

• czyścić = to clean
• wyczyścić = to clean out / clean thoroughly / finish cleaning

In many cases, prefixes in Polish change both the meaning and the aspect of a verb. Here, wyczyścić is the perfective partner of czyścić.

Other examples of similar perfective verbs are:

• pisać → napisać = to write → to write and finish
• robić → zrobić = to do/make → to do/make and complete
• myć → umyć = to wash → to wash completely

Akwarium is the direct object of the verb, so it is in the accusative case.

However, akwarium is a neuter noun, and for many neuter nouns the nominative and accusative forms are the same.

So:

• nominative: akwarium
• accusative: akwarium

That is why it looks unchanged.

Compare that with a feminine noun, which often changes:

• książka = book
• czytam książkę = I am reading a book

Here the accusative is visibly different.

It is basically an international loanword, related to English aquarium. Polish spelling and pronunciation are slightly adjusted to Polish patterns, but the word is very easy for English speakers to recognize.

Grammatically, it behaves like a regular Polish neuter noun ending in -um.

Yes. Polish word order is more flexible than English word order because case endings show grammatical roles.

The neutral order here is:

• W sobotę muszę wyczyścić akwarium.

But you could also say:

• Muszę w sobotę wyczyścić akwarium.
• Akwarium muszę wyczyścić w sobotę.
• Wyczyścić akwarium muszę w sobotę.

These alternatives are not all equally neutral. Changing the order usually changes emphasis:

• W sobotę... emphasizes when
• Akwarium... emphasizes what
• Muszę... emphasizes the necessity

So the original sentence is a very natural, neutral way to say it.

Polish has no articles, so there is no direct equivalent of a/an/the.

So akwarium can mean:

• an aquarium
• the aquarium

The exact meaning depends on context.

This is normal in Polish:

• Mam samochód. = I have a car
• Gdzie jest samochód? = Where is the car?

The article is understood from the situation.

A rough English-friendly guide is:

• muszę ≈ MOO-shen
• wyczyścić ≈ vih-CHISH-chitch
• akwarium ≈ ah-KVAH-ryoom

A few useful pronunciation notes:

• sz sounds like English sh
• cz sounds like English ch
• ść / ści is a soft sound, roughly like shch or a very soft shtch
• w in Polish sounds like English v
• ę is a nasal vowel; at the end of a word, many speakers pronounce it somewhat like -en or -e

So w sobotę muszę wyczyścić akwarium will not sound exactly like English spelling suggests.

Polish sometimes uses we instead of w for easier pronunciation, especially before certain consonant clusters.

For example:

• we wtorek = on Tuesday
• we Wrocławiu = in Wrocław

But here w sobotę is easy to pronounce, so simple w is used.

This alternation is similar to how some languages slightly change short words to avoid awkward sound combinations.

Not naturally in this exact form.

W sobotę usually means this Saturday / on Saturday as a specific occasion, depending on context.

If you want a habitual meaning like on Saturdays, Polish more naturally uses:

• W soboty muszę czyścić akwarium.

Why?

• w sobotę = on Saturday, one occasion
• w soboty = on Saturdays, regularly

And for habits, the imperfective verb czyścić is often more natural than perfective wyczyścić.

So:

• W sobotę muszę wyczyścić akwarium. = On Saturday I have to clean the aquarium.
• W soboty muszę czyścić akwarium. = On Saturdays I have to clean the aquarium.

Yes, it is natural. But depending on context, other verbs are also possible.

For example:

• wyczyścić akwarium = clean the aquarium thoroughly
• umyć akwarium = wash the aquarium
• posprzątać akwarium = tidy up / clean up the aquarium

The best choice depends on what exactly you mean:

• removing dirt thoroughly → wyczyścić
• literally washing → umyć
• general cleaning/tidying → posprzątać

So wyczyścić akwarium is a strong, natural choice when you mean a proper complete cleaning.