Mój współlokator wyprowadza się w sobotę, a nowa współlokatorka wprowadza się w poniedziałek.

Polish nouns usually have grammatical gender, and words for people often change depending on whether the person is male or female.

• współlokator = a male roommate / flatmate
• współlokatorka = a female roommate / flatmate

In this sentence:

• Mój współlokator = my male roommate
• nowa współlokatorka = a new female roommate

The endings help show gender:

• masculine: -or
• feminine: -orka

The adjective or possessive word has to match the noun in gender, number, and case.

Here:

• mój matches the masculine noun współlokator
• nowa matches the feminine noun współlokatorka

So:

• mój współlokator = masculine
• nowa współlokatorka = feminine

If the noun changed, the form would change too. For example:

• moja współlokatorka = my female roommate
• nowy współlokator = a new male roommate

These are two related verbs with different prefixes:

• wyprowadzać się / wyprowadzić się = to move out
• wprowadzać się / wprowadzić się = to move in

The core verb is related to prowadzić = to lead, to guide, to conduct.
The prefixes change the direction:

• wy- often suggests movement out
• w- often suggests movement in

So:

• wyprowadza się = is moving out / moves out
• wprowadza się = is moving in / moves in

They are a natural pair in Polish, just like move out and move in in English.

Because in Polish, wyprowadzać się and wprowadzać się are reflexive-style verbs used with się when talking about a person changing where they live.

Without się, the verb can mean something else or sound incomplete in this context.

Compare:

• wyprowadzać się = to move out
• wprowadzać się = to move in

The się is just part of how these verbs are normally used when talking about someone relocating themselves.

English learners often want to translate się literally as oneself, but that usually does not help much. Here it is better to learn the whole expression:

• wyprowadzać się
• wprowadzać się

as complete verb phrases.

Polish often uses the present tense of an imperfective verb to talk about a planned or scheduled future event.

So:

• wyprowadza się w sobotę literally looks like is moving out on Saturday
• wprowadza się w poniedziałek literally looks like is moving in on Monday

But in natural English, the meaning is future:

• is moving out on Saturday
• is moving in on Monday

This is very similar to English sentences like:

• My roommate leaves on Saturday.
• She starts work on Monday.

So the present form in Polish can express a future event when the time is clearly stated.

Here w means on, when talking about days.

After w in this time expression, Polish uses the accusative case.

So:

• sobota → w sobotę
• poniedziałek → w poniedziałek

This is why the forms are not both the basic dictionary form.

A useful pattern to remember:

• w sobotę = on Saturday
• w niedzielę = on Sunday
• w poniedziałek = on Monday
• we wtorek = on Tuesday

Some days visibly change more than others.

Because different noun types form the accusative differently.

1. Feminine noun: sobota

Most feminine nouns ending in -a change that ending to -ę in the accusative singular:

• sobota → sobotę

2. Masculine noun: poniedziałek

For many masculine inanimate nouns, the accusative singular is the same as the nominative:

• poniedziałek → poniedziałek

So even though both are in the accusative after w, only sobota shows a visible change here.

In Polish, a often connects two ideas with a sense of contrast or comparison, even if the contrast is mild.

Here the sentence is balancing two related events:

• one roommate moves out on Saturday,
• a new roommate moves in on Monday.

So a works very naturally because it sets the second fact against the first:

• Mój współlokator wyprowadza się w sobotę, a nowa współlokatorka wprowadza się w poniedziałek.

If you used i, it would simply mean and, with less of that balancing effect.
Both ideas are connected, but a sounds more natural here because one person is leaving and another is arriving.

Yes. In Polish, just like in English, you do not always need to state the place if it is already understood from context.

So:

• wyprowadza się = is moving out
• wprowadza się = is moving in

If needed, you can add more detail:

• wyprowadza się z mieszkania = he is moving out of the apartment
• wprowadza się do mieszkania = she is moving into the apartment

But in your sentence, the apartment or home is already implied, so leaving it unstated is completely natural.

Because Polish often leaves out words that are obvious from context.

In this sentence, once you hear:

• Mój współlokator = my roommate

it is already easy to understand that the new female roommate is also connected to the same living situation. So saying simply:

• nowa współlokatorka

sounds natural.

You could say:

• moja nowa współlokatorka

but it would feel a bit more explicit. The original sentence is smoother and more natural in ordinary conversation.

The forms wyprowadza się and wprowadza się are imperfective.

The verb pairs are:

• wyprowadzać się (imperfective) / wyprowadzić się (perfective)
• wprowadzać się (imperfective) / wprowadzić się (perfective)

In this sentence, the imperfective present is used for a planned future event:

• wyprowadza się w sobotę
• wprowadza się w poniedziałek

This is very common in Polish.

If you used the perfective future, it would also be grammatical, but slightly different in feel:

• Mój współlokator wyprowadzi się w sobotę...
• ...nowa współlokatorka wprowadzi się w poniedziałek.

That version can sound a bit more like a completed future fact. The imperfective present often sounds more like an arranged plan or scheduled event.

Not completely. Polish word order is fairly flexible, because endings carry a lot of grammatical information.

The original sentence is very natural:

• Mój współlokator wyprowadza się w sobotę, a nowa współlokatorka wprowadza się w poniedziałek.

But other orders are possible for emphasis, for example:

• W sobotę mój współlokator wyprowadza się, a w poniedziałek nowa współlokatorka wprowadza się.

This version emphasizes the days.

Even so, the original order is probably the most neutral and natural for a learner to use.

The prefix współ- means something like co- or shared with someone else.

So:

• współlokator = co-tenant / roommate / flatmate
• literally, someone who lives in the same place as another person

Pronunciation can be tricky for English speakers because of the consonant cluster at the start:

• współ-

A rough approximation is something like fspoo-w at the beginning, though not exactly. The ó sounds like u, and Polish stress usually falls on the second-to-last syllable:

• współloKAter? No
• współloKAtor? Also no
• correct stress: współloKAtor? Actually yes, because the penultimate syllable is ka
• współlokaTORka? No, in współlokatorka the stress is on tor? Let's count: współ-lo-ka-tor-ka → the second-to-last syllable is tor, so współlokaTORka

So:

• współloKAtor
• współlokaTORka

Learners often find these words hard at first, but they are very common and worth practicing as whole chunks.

Not really, if you specifically mean roommate / flatmate.

• współlokator / współlokatorka is the normal word for someone you live with.
• kolega means something more like male friend, buddy, or colleague, depending on context.

So kolega z mieszkania would sound unusual and less precise. If you mean a person who shares your flat or apartment, współlokator / współlokatorka is the best choice.