Na tržnici sam kupila svježe brašno, pa ću večeras opet mijesiti tijesto.

Both are possible. Croatian word order is flexible, and short unstressed words like sam often come in the second position of the clause.

So:

• Na tržnici sam kupila svježe brašno.
• Kupila sam svježe brašno na tržnici.

Both mean the same thing, but the first version puts focus first on where it happened: at the market.

Also, sam is the auxiliary used to form the past tense.

Because the speaker is female.

In the Croatian past tense, the main verb agrees with the subject's gender and number:

• kupio = a man bought
• kupila = a woman bought
• kupilo = it bought
• kupili = they bought (masculine mixed group)
• kupile = they bought (all feminine)

So sam kupila tells you that the speaker is a woman saying I bought.

Because na tržnici means at the market, and after na when talking about location, Croatian uses the locative case.

Base noun:

• tržnica = market

Locative singular:

• tržnici

So:

• na tržnici = at the market

Compare:

• na tržnicu = to the market (motion, accusative)
• na tržnici = at the market (location, locative)

Because brašno is a neuter singular noun, and the adjective has to agree with it.

• svjež = masculine
• svježa = feminine
• svježe = neuter

So:

• svježe brašno = fresh flour

This is adjective-noun agreement in gender, number, and case.

It is in the accusative, but for many neuter singular inanimate nouns, the accusative looks the same as the nominative.

So:

• nominative: brašno
• accusative: brašno

That is normal.

The same thing happens with tijesto:

• nominative: tijesto
• accusative: tijesto

Here pa means something like:

• so
• and so
• therefore
• then

It links the two parts of the sentence naturally:

• Na tržnici sam kupila svježe brašno, pa ću večeras opet mijesiti tijesto.
• I bought fresh flour at the market, so I'll be making/kneading dough again tonight.

It often sounds more conversational and flowing than a direct equivalent like zato.

Because Croatian future tense is often formed with:

• a short form of htjeti (ću, ćeš, će, ćemo, ćete, će)
• plus the infinitive

So:

• ću mijesiti = I will knead / I’ll be kneading

The short auxiliary ću is an unstressed word, so it usually appears near the beginning of the clause, often in second position.

That is why you get:

• pa ću večeras opet mijesiti tijesto

rather than keeping the whole verbal idea together the way English does.

Mijesiti means to knead or to mix into dough.

In this sentence, it fits very naturally with tijesto:

• mijesiti tijesto = to knead/make dough

A native English translation might vary depending on context:

• I’ll knead dough again tonight
• I’ll be making dough again tonight

So even if the literal meaning is closer to knead, in natural English it may be translated more loosely.

Because Croatian often drops the subject pronoun when it is already clear from the verb form.

Here the verb forms already show the subject:

• sam kupila = I bought
• ću mijesiti = I will knead

So ja is not necessary.

You could say Ja sam kupila..., but that would add emphasis, like I bought it....

Večeras means tonight / this evening.

It is an adverb of time. It tells you when the speaker will knead the dough.

So:

• ću večeras opet mijesiti tijesto = I’ll be kneading dough again tonight

It does not change form here.

Opet means again.

In this sentence:

• pa ću večeras opet mijesiti tijesto

it means the speaker has done this before and will do it one more time.

Because Croatian word order is flexible, opet can move around somewhat:

• Pa ću večeras opet mijesiti tijesto.
• Pa ću opet večeras mijesiti tijesto.

The meaning stays similar, though the emphasis can shift slightly.

They are different things:

• brašno = flour
• tijesto = dough

So the sequence makes sense:

1. The speaker bought fresh flour
2. Therefore, they will knead/make dough

This is a good example of how Croatian uses very specific everyday food vocabulary.

Because pa connects two clauses here:

• Na tržnici sam kupila svježe brašno
• pa ću večeras opet mijesiti tijesto

In Croatian, it is normal to put a comma before pa when it links clauses in this way, especially when the second clause expresses a result or continuation.

Yes, it can be natural, especially in context. Svježe often means fresh, but in food contexts it can also suggest something like:

• recently milled
• newly bought
• good and fresh-quality

So svježe brašno is understandable and natural, especially if the point is that the flour is suitable for baking right away.

Yes, depending on what the speaker wants to emphasize.

• mijesiti is imperfective: it focuses on the process, repeated action, or general activity
• umijesiti is perfective: it focuses on completing the dough-making

So:

• opet mijesiti tijesto = knead/make dough again, focusing on the activity
• opet ću umijesiti tijesto = I’ll make/knead the dough again, with more emphasis on finishing it

In your sentence, mijesiti sounds very natural because it fits the idea of what the speaker plans to do this evening as an activity.