Autobus przyjedzie za piętnaście minut, więc możemy jeszcze spokojnie porozmawiać.

Przyjedzie is a conjugated verb form: it means will arrive.
Przyjechać is the infinitive: to arrive / to come (by vehicle).

In a full sentence, Polish usually needs a finite verb, just like English does:

• Autobus przyjedzie... = The bus will arrive...

You would only use przyjechać in structures like:

• Autobus może przyjechać = The bus may arrive
• Chcę przyjechać = I want to come/arrive

So here przyjedzie is the correct form because it is the main verb of the sentence.

That is because przyjechać is a perfective verb. In Polish, perfective verbs do not normally have a true present tense. Their present-looking forms refer to the future.

So:

• przyjeżdża = is coming / comes (imperfective, present)
• przyjedzie = will come / will arrive (perfective, future)

The idea is that przyjechać describes a completed event: the bus will reach its destination. That is why Polish naturally uses the perfective future here.

Yes. In time expressions, za + accusative often means in ... time, meaning after a certain amount of time from now.

So:

• za minutę = in a minute
• za godzinę = in an hour
• za piętnaście minut = in fifteen minutes

This is a very common Polish pattern.

This is a very common learner question.

• za piętnaście minut = in fifteen minutes = the event will happen 15 minutes from now
• w piętnaście minut = in fifteen minutes in the sense of within that amount of time / taking that long to do something

Compare:

• Autobus przyjedzie za piętnaście minut. = The bus will arrive 15 minutes from now.
• Napisałem to w piętnaście minut. = I wrote it in 15 minutes.

So in your sentence, za is the only correct choice.

After the numeral piętnaście (fifteen), the noun usually appears in the genitive plural.

That is why:

• pięć minut = five minutes
• dziesięć minut = ten minutes
• piętnaście minut = fifteen minutes

The singular minuta changes to minut in this pattern.

This is one of the standard numeral rules in Polish:

• 1 usually takes singular
• 2, 3, 4 often take nominative/accusative plural
• 5 and higher usually take genitive plural

Więc means so, therefore, or thus, depending on context. In everyday speech, so is usually the best translation.

Here it links the two ideas:

• The bus will arrive in fifteen minutes,
• so we can still talk calmly.

It is a very natural, common connector in Polish conversation.

Możemy is present tense, but in Polish, just like in English, the present can sometimes express a current possibility about the near future.

So:

• możemy jeszcze porozmawiać = we can still talk

  This means the opportunity exists now, because the bus has not arrived yet.

If you said będziemy mogli, that would mean we will be able to, which sounds more explicitly future-oriented. It is not wrong in some contexts, but here możemy is more natural because the possibility is already available right now.

Here jeszcze means still or yet in the sense of there is still time.

So:

• możemy jeszcze... = we can still...

It suggests that the situation has not reached its limit yet. The bus is not here yet, so there is still time to do something before it arrives.

Spokojnie is an adverb from spokojny (calm, peaceful). Here it means something like:

• calmly
• without rushing
• without stress
• at ease

In this sentence, it suggests: we don’t need to hurry; we can talk in a relaxed way.

So spokojnie porozmawiać is not just talk, but talk without pressure or rush.

This is about aspect again.

• rozmawiać = imperfective = to be talking / to talk in a general or ongoing sense
• porozmawiać = perfective = to have a talk / to chat for a bit, seen as a complete, bounded event

In this sentence, porozmawiać sounds natural because the speaker means have some conversation before the bus comes. It is treated as a limited, complete action.

Compare:

• Lubimy rozmawiać. = We like talking.
• Możemy jeszcze porozmawiać. = We can still have a chat / talk a bit more.

After modal verbs such as móc (can / to be able to), Polish normally uses the infinitive of the main action.

So:

• mogę mówić = I can speak
• możesz przyjść = you can come
• możemy porozmawiać = we can talk / have a chat

That is exactly the same basic pattern as in English: can + base verb.

Yes, Polish word order is fairly flexible, although some orders sound more natural than others.

The original sentence:

• Autobus przyjedzie za piętnaście minut, więc możemy jeszcze spokojnie porozmawiać.

is very neutral and natural.

You could also hear variations such as:

• Za piętnaście minut autobus przyjedzie, więc możemy jeszcze spokojnie porozmawiać.
• Możemy jeszcze spokojnie porozmawiać, bo autobus przyjedzie za piętnaście minut.

The meaning stays similar, but the emphasis changes:

• starting with za piętnaście minut emphasizes the time
• starting with możemy emphasizes the possibility

So yes, the order can change, but the original version is a very standard one.

In this sentence, yes, autobus is needed because it gives the concrete subject: the bus.

Polish often drops pronoun subjects like ja, ty, my, because the verb ending already shows who is doing the action. But nouns are different. If you removed autobus, then przyjedzie za piętnaście minut would just mean it/he/she will arrive in fifteen minutes, and the listener would need to know from context who or what that is.

So the noun autobus is important here unless the bus has already been clearly established in the conversation.