Moja siostra zakłada sukienkę na spotkanie, a później przebiera się w ciepłą bluzę.

Because siostra is feminine, singular, and in the nominative case, so the possessive adjective must match it.

Polish adjectives and possessives agree with the noun in:

• gender
• number
• case

So:

• mój brat = my brother
• moja siostra = my sister
• moje dziecko = my child

They are in the accusative singular, not the basic dictionary form.

In this sentence:

• zakłada sukienkę — the dress is the direct object
• przebiera się w bluzę — the clothing after w is also in the accusative here

For many feminine nouns ending in -a, the accusative singular changes to -ę:

• sukienka → sukienkę
• bluza → bluzę

That is why you do not see the nominative forms sukienka and bluza.

Because the adjective must agree with the noun in case, gender, and number.

Since bluzę is feminine singular accusative, the adjective must also be feminine singular accusative:

• ciepła bluza = nominative
• ciepłą bluzę = accusative

So the whole phrase changes together.

Not exactly.

zakładać means to put on a specific item of clothing, footwear, glasses, etc.

So:

• zakłada sukienkę = she puts on a dress

Compare:

• ubiera się = she gets dressed
• nosi sukienki = she wears dresses, or she tends to wear dresses
• ma na sobie sukienkę = she is wearing a dress

So zakładać focuses on the action of putting something on.

Here na spotkanie means something like for the meeting or to the meeting.

The preposition na often takes the accusative when it shows:

• destination
• purpose
• an event someone is going to

So:

• na spotkanie = for/to the meeting
• na koncert
• na obiad
• na imprezę

A useful contrast:

• na spotkanie = to/for the meeting
• na spotkaniu = at the meeting

Both can often translate as and, but they are not always used the same way.

• i simply joins things more tightly
• a often links two clauses with a slight contrast, shift, or change of scene

In this sentence, a później sounds natural because the second action is a new stage:

• first she puts on a dress
• later she changes into a warm sweatshirt

So a here is very natural English-wise as and then or while/whereas, depending on context.

przebiera się means changes clothes or changes into something else.

The word się is the reflexive particle. It often shows that the action is done to oneself.

So:

• przebiera się = she changes herself / she changes clothes

Very useful comparison:

• przebiera się w bluzę = she changes into a sweatshirt
• przebiera się za lekarza = she dresses up as a doctor

So:

• w + accusative = into a piece of clothing
• za + accusative = as, in the role/costume of

That is a very common question.

The preposition w can take different cases:

• locative when it means location: in, inside
• accusative when it suggests movement into something, or with certain verbs such as changing into clothes

Here, with przebierać się w, the meaning is change into, so Polish uses the accusative:

• w ciepłą bluzę = into a warm sweatshirt

Compare:

• Jest w ciepłej bluzie. = She is in a warm sweatshirt / She is wearing a warm sweatshirt.
• Przebiera się w ciepłą bluzę. = She changes into a warm sweatshirt.

So the difference is basically:

• location/state → w ciepłej bluzie
• change into → w ciepłą bluzę

No — this is a false friend.

bluza usually means:

• sweatshirt
• hoodie
• casual warm top

If you want blouse, the usual Polish word is bluzka.

So:

• bluza = sweatshirt/hoodie
• bluzka = blouse

Most naturally, yes — it sounds like a general description, a routine, or a present-tense narration.

Both verbs are imperfective:

• zakładać
• przebierać się

In the present tense, imperfective verbs can describe:

• habitual actions
• repeated actions
• actions seen as ongoing or in progress, depending on context

Also, Polish does not have a separate tense exactly like English is putting on. The present tense often covers both puts on and is putting on, depending on context.

If you wanted to emphasize a single completed future sequence, you would more likely use perfective forms:

• założy
• przebierze się

Because Polish does not have articles.

So sukienkę can mean:

• a dress
• the dress

And na spotkanie can mean:

• for a meeting
• for the meeting

Polish usually leaves this to context. Native speakers understand whether something is definite or indefinite from the situation, not from separate words like a or the.