Czekaliśmy długo na autobus, ale wreszcie przyjechał i wszyscy wsiedli.

Because Polish often leaves subject pronouns out when they are already clear from the verb form.

Czekaliśmy already tells you:

• past tense
• we
• plural

So my czekaliśmy is possible, but usually unnecessary unless you want extra emphasis on we.

It can mean either, depending on context.

Czekaliśmy is the past tense of the imperfective verb czekać. Polish imperfective past does not force the same distinction English makes between:

• we waited
• we were waiting

In this sentence, the idea is an ongoing action before another event happened, so English often uses we waited a long time or we were waiting a long time.

This is a very common Polish aspect pattern.

• czekaliśmy = ongoing/background action
• przyjechał = completed event
• wsiedli = completed event

So the sentence works like this:

1. We were in the middle of waiting.
2. Then the bus arrived.
3. Then everyone got on.

Polish often uses an imperfective verb for the background situation and perfective verbs for the main events that move the story forward.

After czekać, Polish normally uses na + accusative:

• czekać na kogoś / na coś = to wait for someone / something

So:

• na autobus = for the bus

Here autobus is in the accusative singular.

Because autobus is a masculine inanimate noun, and for many masculine inanimate nouns, the accusative singular has the same form as the nominative singular.

So:

• nominative: autobus
• accusative: autobus

That is why you do not see a visible change here, even though the case has changed.

Przyjechał comes from przyjechać, which means to arrive by vehicle, to come by riding, or simply to arrive when the subject is something like a bus, train, or car.

With autobus, this is the natural verb:

• a bus przyjechał
• a person on foot would usually przyszedł
• a person arriving by vehicle could also przyjechał

So here przyjechał is the normal way to say that the bus arrived.

Because Polish grammar uses grammatical gender, not real-life biological sex.

Autobus is a masculine noun, so in the past tense the verb agrees with it:

• autobus przyjechał

That does not mean the bus is male in any real-world sense. It is just grammatical agreement.

Wreszcie means finally or at last.

It is very close to w końcu, and in many contexts they are interchangeable. The difference is mostly one of style and nuance:

• wreszcie = slightly more literary or neat
• w końcu = very common in everyday speech

Another similar word is nareszcie, which often sounds a bit more emotional, like finally! with relief or impatience.

Wszyscy is the masculine-personal plural form meaning everyone / all people.

Polish uses this form for a group of people when:

• the group includes at least one male, or
• the group is mixed, or
• the group is treated generically as people

If the group were all female, you would normally expect:

• wszystkie

So wszyscy tells you that the group is being treated as masculine-personal.

Wsiedli is the masculine-personal plural past form, matching wszyscy.

So:

• wszyscy wsiedli = masculine-personal plural agreement

This matches the grammar of the subject. Polish past tense verbs agree in gender/number in the plural too, not just in the singular.

Because wsiąść has somewhat irregular past forms, and you mostly have to learn them as a pattern.

Useful forms are:

• wsiadł = he got in / boarded
• wsiadła = she got in / boarded
• wsiedli = they got in / boarded, masculine-personal
• wsiadły = they got in / boarded, non-masculine-personal

So wsiedli is the correct form here because the subject is wszyscy.

Because Polish normally uses wsiąść / wsiadać for boarding a vehicle.

So:

• wsiąść do autobusu = to get on the bus / board the bus

You can say weszli do autobusu, but that focuses more literally on the physical act of walking into the bus. Wsiedli is the standard, idiomatic choice for getting on a vehicle.

That fuller version is also correct.

When the vehicle is stated explicitly, Polish usually says:

• wsiąść do + genitive
• wsiąść do autobusu

In your sentence, do autobusu is left out because the context already makes it obvious which vehicle everyone boarded.

No. Polish word order is fairly flexible.

The given sentence is natural, but other orders are possible, for example:

• Długo czekaliśmy na autobus...
• Na autobus czekaliśmy długo...

These versions do not radically change the meaning, but they shift emphasis. The original order sounds natural and neutral.

Because in Polish, ale is normally preceded by a comma when it connects clauses.

So:

• Czekaliśmy długo na autobus, ale wreszcie przyjechał...

This is standard punctuation, just like a comma before but in many English sentences.