Nawet zimą trzeba czasem wietrzyć mieszkanie, ale tylko przez chwilę.

Because zimą is the instrumental singular of zima.

In Polish, names of seasons are often used in the instrumental to mean in winter / in spring / in summer / in autumn:

• zimą = in winter
• wiosną = in spring
• latem = in summer
• jesienią = in autumn

So Nawet zimą... means Even in winter...

You may also hear w zimie, but zimą is very common and natural.

Nawet means even.

In this sentence, it emphasizes that something is true in a situation where you might not expect it:

• Nawet zimą trzeba... = Even in winter, it is necessary...

So the idea is: although winter is cold, you still sometimes need to air out the apartment.

Trzeba is an impersonal word meaning something like:

• it is necessary
• one must
• you need to
• people should

Polish often leaves this kind of statement without a specific subject. So:

• Trzeba wietrzyć mieszkanie = You need to air out the apartment / One should air out the apartment

There is no explicit you, we, or people, but that idea is understood from context.

Because trzeba is followed by an infinitive.

This is a very common Polish pattern:

• trzeba iść = one must go
• trzeba pracować = one must work
• trzeba wietrzyć = one must air out

So trzeba czasem wietrzyć mieszkanie literally follows the pattern it is necessary sometimes to air the apartment.

Not exactly.

Wietrzyć means to air out, to ventilate, or to let fresh air into a room or apartment. It often implies opening windows, but the focus is on refreshing the air, not just the physical act of opening a window.

So:

• otworzyć okno = to open a window
• wietrzyć mieszkanie = to air out the apartment

Mieszkanie is the direct object of wietrzyć, so it is in the accusative case.

However, for many neuter singular nouns, the nominative and accusative forms look the same.

So:

• nominative: mieszkanie
• accusative: mieszkanie

That is why the form does not change visibly here.

Czasem means sometimes.

It is very close in meaning to czasami, which also means sometimes. In most everyday situations, they are interchangeable.

So you could say:

• trzeba czasem wietrzyć mieszkanie
• trzeba czasami wietrzyć mieszkanie

Both are natural. Czasem is just a bit shorter and very common.

Because przez + accusative is often used in Polish to express duration.

Here:

• chwila = moment
• chwilę = accusative singular
• przez chwilę = for a moment / for a short while

So ale tylko przez chwilę means but only for a short time.

This is a useful pattern:

• przez godzinę = for an hour
• przez kilka dni = for several days
• przez moment / przez chwilę = for a moment

Because after przez, Polish usually uses the accusative case.

The base form is:

• chwila = a moment

But after przez, it becomes:

• przez chwilę = for a moment

So the ending changes because of the case required by the preposition.

Wietrzyć is imperfective, and that fits well because the sentence is talking about a general habit or repeated necessity, not one completed action.

The idea is:

• from time to time, you should air out the apartment

Polish usually uses the imperfective for habits, routines, and general advice.

A perfective verb would sound more like a single completed airing-out. In other contexts, you might hear verbs like przewietrzyć or wywietrzyć, but here wietrzyć is the natural choice for a general statement.

No, Polish word order is fairly flexible, but the chosen order sounds natural and gives the right emphasis.

Nawet zimą at the beginning highlights the surprising part first: even in winter.

The sentence as written flows like this:

• Nawet zimą = even in winter
• trzeba czasem = it is sometimes necessary
• wietrzyć mieszkanie = to air out the apartment
• ale tylko przez chwilę = but only for a short while

You could rearrange parts of it, but the emphasis would change. For example, moving nawet zimą later would make it less prominent.

It adds a contrast or limitation:

• yes, you should air out the apartment
• but only briefly

So ale means but, and tylko means only.

This softens the advice and makes it more precise. In winter, airing out is still recommended, but not for a long time.