Gdyby autobus nie przyjechał, musiałbym iść pieszo do pracy.

Gdyby introduces a hypothetical condition. In this sentence, it means if in the sense of if the bus didn’t arrive / if the bus were not to arrive.

It is used for unreal or imagined situations. That is different from a more neutral condition with jeśli or jeżeli, which often corresponds to ordinary if in English.

So the structure is:

• Gdyby autobus nie przyjechał = If the bus didn’t arrive
• musiałbym iść pieszo do pracy = I would have to walk to work

This is one of the most common things that confuses English speakers.

After gdyby, Polish normally uses a past form of the verb, even when the meaning is hypothetical rather than truly past. So:

• gdyby ... przyjechał
  literally looks like if ... arrived / had arrived
• but in meaning it often works like English if ... didn’t come / were not to come

So Polish forms unreal conditions differently from English. The form is past, but the meaning is conditional/hypothetical.

Also, przyjechał agrees with autobus, which is masculine singular, so the masculine singular form is used.

This is about aspect.

• przyjechać is perfective
• przyjeżdżać is imperfective

Here, the speaker is talking about one specific completed arrival of the bus. That is why Polish uses the perfective verb przyjechać, in the form przyjechał.

So:

• nie przyjechał = did not arrive / did not come
• nie przyjeżdżał would suggest something more like wasn’t coming regularly, wasn’t in the habit of coming, or in some contexts an ongoing/repeated action

For a single bus trip, przyjechał is the natural choice.

Because autobus is the subject of the clause, and subjects are normally in the nominative case.

Here:

• autobus = the bus
• autobus nie przyjechał = the bus did not arrive

The negation nie affects the verb, but it does not change the subject into another case.

English speakers sometimes expect negation to change noun forms because they may have heard about genitive with negation in Polish. But that mainly applies to some objects, not to subjects like autobus here.

It means did not arrive or did not come.

The verb przyjechać specifically suggests arriving by vehicle or coming from elsewhere by transport. Since the subject is autobus, that fits perfectly:

• autobus przyjechał = the bus arrived
• autobus nie przyjechał = the bus did not arrive

The negative particle nie goes directly before the verb.

Musiałbym means I would have to.

It is made from:

• musiał = past form of musieć = to have to
• bym = conditional particle marking I would

So:

• musiałbym = I would have to
• musiałbyś = you would have to
• musiałby = he/it would have to
• musiałaby = she would have to

This is a very common Polish way to form the conditional.

Because the whole sentence is hypothetical.

• muszę = I have to
• musiałbym = I would have to

Since the first clause says if the bus didn’t arrive, the result is also hypothetical, so Polish uses the conditional in the main clause:

• Gdyby autobus nie przyjechał, muszę iść... would sound wrong
• Gdyby autobus nie przyjechał, musiałbym iść... is correct

So the pattern is:

• hypothetical condition
• hypothetical result

Because iść is used for one specific trip in one direction, while chodzić is more about habit, repetition, or movement in general.

Here the meaning is:

• I would have to go/walk to work on that occasion

So iść is the natural choice.

Compare:

• musiałbym iść do pracy = I would have to go to work once, in that situation
• musiałbym chodzić do pracy pieszo = I would have to walk to work regularly / as a habit

Pieszo is an adverb, and it means on foot.

So:

• iść pieszo = to go on foot
• do pracy pieszo = to work on foot

It is not a case form of a noun. It is just an adverb describing how you go.

A common alternative is:

• na piechotę

So both can mean on foot, though pieszo is a little more neutral and compact.

Because do + genitive is the standard way to express movement to many destinations.

Here:

• praca = work
• pracy = genitive singular
• do pracy = to work

So:

• iść do pracy = to go to work
• jechać do pracy = to go to work by some vehicle

Compare:

• w pracy = at work
• z pracy = from work

English speakers often want to translate word-for-word, but do pracy is simply the normal Polish expression for to work.

A woman would say:

Gdyby autobus nie przyjechał, musiałabym iść pieszo do pracy.

The only change is:

• musiałbym → musiałabym

That is because the speaker is female, and in Polish the past and conditional forms often show the speaker’s gender.

The verb in the first clause, przyjechał, does not change here, because it agrees with autobus, which is masculine.

Yes. Polish word order is fairly flexible.

You can also say:

Musiałbym iść pieszo do pracy, gdyby autobus nie przyjechał.

That still means the same thing: I would have to walk to work if the bus didn’t arrive.

The difference is mainly one of emphasis and flow:

• starting with Gdyby... foregrounds the condition
• starting with Musiałbym... foregrounds the consequence

But some things stay fixed:

• nie normally goes right before the verb
• the overall grammar of the conditional stays the same

Because Polish normally separates a subordinate clause from the main clause with a comma.

Here, Gdyby autobus nie przyjechał is the conditional subordinate clause, and musiałbym iść pieszo do pracy is the main clause.

So the comma is standard and expected:

• Gdyby autobus nie przyjechał, musiałbym iść pieszo do pracy.

If you reverse the order, you still use a comma:

• Musiałbym iść pieszo do pracy, gdyby autobus nie przyjechał.

Yes, that is the closest match.

It works like English sentences such as:

• If the bus didn’t come, I would have to walk to work.

So in broad terms, this is an unreal/hypothetical conditional.

But the grammar does not match English exactly. Polish uses:

• gdyby
  • past-form verb in the conditional clause
• conditional form in the main clause

So even though it is similar in meaning to the English second conditional, it is built in a characteristically Polish way.