W marcu pogoda często się zmienia, więc rano biorę ciepłą kurtkę.

Because after w meaning in for a month, Polish uses the locative case.

• marzec = March
• w marcu = in March

This is very common with months:

• w styczniu = in January
• w lutym = in February
• w marcu = in March

So w marcu is just the correct case form after w in this time expression.

Marcu is the locative singular form of marzec.

The dictionary form is:

• marzec = March

But in the sentence, Polish changes the noun ending because of the preposition w:

• w marcu = in March

Many Polish nouns change form after prepositions, so this is a normal case ending, not a separate word to memorize on its own.

In Polish, the verb is often used as zmieniać się or zmienić się when something changes.

So:

• zmieniać can mean to change something
• zmieniać się means to change / to be changing oneself, in the sense of undergoing change

Here, the weather is not changing something else; the weather itself changes, so Polish uses:

• pogoda się zmienia = the weather changes / is changing

This się often appears in verbs where English does not use a reflexive word.

Zmienia is:

• 3rd person singular
• present tense
• from the imperfective verb zmieniać się

It agrees with pogoda because pogoda is singular:

• pogoda się zmienia = the weather changes / is changing

A quick comparison:

• ja się zmieniam = I change / am changing
• ty się zmieniasz = you change
• pogoda się zmienia = the weather changes

Często means often and works as an adverb.

In this sentence:

• pogoda często się zmienia

it modifies the verb phrase się zmienia = changes.

This word order is very natural in Polish. Polish word order is more flexible than English, but this version sounds neutral and standard.

You may also hear other orders in different contexts, but pogoda często się zmienia is the most straightforward.

Because więc means so, therefore, or thus, and in Polish it is normally separated by a comma when it connects clauses.

So:

• W marcu pogoda często się zmienia, więc rano biorę ciepłą kurtkę.

This is standard Polish punctuation.

Więc means something like:

• so
• therefore
• that’s why

It shows a result or conclusion.

In this sentence:

• The weather often changes in March, so in the morning I take a warm jacket.

So the structure is:

• situation: pogoda często się zmienia
• result: więc rano biorę ciepłą kurtkę

Because rano is an adverb meaning in the morning / in the morning time.

So you simply say:

• rano = in the morning

Examples:

• Rano piję kawę. = I drink coffee in the morning.
• Rano jest chłodno. = It is cool in the morning.

You do not say w rano.

A related expression is:

• rano = in the morning
• wieczorem = in the evening
• nocą = at night

Biorę is the 1st person singular present form of brać = to take.

So:

• biorę = I take / I am taking

In this sentence, it suggests a habitual action:

• rano biorę ciepłą kurtkę = in the morning I take a warm jacket

In natural English, you might translate this more idiomatically as I take a warm jacket with me or sometimes even I grab a warm jacket.

Polish often uses brać where English might use a slightly different everyday verb.

Because biorę is imperfective, and it fits a repeated or habitual action.

• brać → imperfective
• wziąć → perfective

So:

• biorę = I take / I usually take
• wezmę = I will take / I’ll take once

Since the sentence describes what the speaker generally does because March weather is changeable, biorę is the natural choice.

If you said:

• więc rano wezmę ciepłą kurtkę

that would sound more like so I’ll take a warm jacket this morning / on this occasion, not a general habit.

Because kurtkę is the direct object of biorę, so it must be in the accusative case.

Dictionary form:

• ciepła kurtka = a warm jacket

In the accusative singular:

• ciepłą kurtkę

Why both words change:

• kurtka is a feminine noun
• ciepła is an adjective that must agree with it

So:

• nominative: ciepła kurtka
• accusative: ciepłą kurtkę

This is a very important pattern for feminine singular nouns.

Because it answers the question what do I take?

• Biorę co? → ciepłą kurtkę

That makes it the direct object, and direct objects after many verbs go into the accusative.

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

• kurtka → kurtkę
• książka → książkę
• herbata → herbatę

And the adjective changes too:

• ciepła → ciepłą

Yes, but the meaning changes a little.

• biorę ciepłą kurtkę = I take a warm jacket / I take a warm jacket with me
• zakładam ciepłą kurtkę = I put on a warm jacket

So biorę focuses on taking it, while zakładam focuses on putting it on.

Depending on context, either could make sense. If the speaker means they actually wear it right away, zakładam may be more precise. If they mean they take it along because the weather may change later, biorę is very natural.

Because Polish usually does not need the subject pronoun if the verb ending already shows who the subject is.

• biorę clearly means I take

So ja is not necessary.

You could say:

• więc rano ja biorę ciepłą kurtkę

but that would usually add emphasis or contrast, something like:

• so I take a warm jacket

In neutral statements, Polish normally leaves ja out.

It most naturally describes a general habit.

Clues:

• często = often
• biorę = imperfective present, often used for repeated actions
• the whole sentence gives a general reason: March weather changes a lot

So the meaning is more like:

• In March, the weather often changes, so I take a warm jacket in the morning.

not just one single occasion.

Yes, Polish word order is flexible, but the original sentence is neutral and natural.

Original:

• W marcu pogoda często się zmienia, więc rano biorę ciepłą kurtkę.

Other possible orders are grammatically possible, for example:

• Rano biorę ciepłą kurtkę, bo w marcu pogoda często się zmienia.
• Ciepłą kurtkę biorę rano, bo w marcu pogoda często się zmienia.

But changing the order changes the focus or emphasis. The original version sounds like a normal, unmarked statement.

Biorę is roughly pronounced like BYO-reh with a nasal ending.

A few useful points:

• bio- sounds like byo
• rę contains ę, which is a nasal vowel
• at the end of a word, ę is often pronounced less strongly nasal than learners expect

So in normal speech, biorę may sound closer to byo-re with some nasal quality, rather than a very strong separate nasal sound.

The same is true for się: in everyday pronunciation, the nasal sound is often lighter than textbooks suggest.

The dictionary forms are:

• zmieniać się = to change
• brać = to take

Forms used in the sentence:

• zmienia ← from zmieniać się
• biorę ← from brać

These are worth learning as a pair with their meanings and aspect:

• zmieniać się / zmienić się
• brać / wziąć

That helps a lot when you start building your own sentences.