Kiedy pada deszcz, wsiadam do autobusu zamiast iść pieszo do pracy.

Here it is best understood as whenever. The sentence describes a repeated habit, not one single occasion:

Kiedy pada deszcz, wsiadam do autobusu...
= Whenever it rains, I get on / take the bus...

Polish often uses the present tense this way for general or repeated situations.

Because the verb already shows the subject.

wsiadam ends in -am, which tells you it means I get on / I board. In Polish, subject pronouns like ja are often omitted unless you want emphasis or contrast.

So:

• Wsiadam do autobusu = normal, neutral
• Ja wsiadam do autobusu = more emphatic, like I get on the bus

Yes. pada deszcz is a standard Polish way to express rain.

Literally, it is something like rain is falling, but in normal English translation it is simply it’s raining.

You may also hear:

• Pada. = It’s raining.
• Pada deszcz. = Rain is falling / It’s raining.

The version with deszcz is a little fuller; the version without it is very common in everyday speech.

Both are possible, but pada deszcz is the more neutral, natural order in this kind of weather statement.

Polish word order is flexible, but it is not random. In simple neutral descriptions, the verb-first pattern is common with weather expressions:

• Pada deszcz.
• Pada śnieg.

Deszcz pada is possible, but it sounds more marked, as if you are emphasizing the rain.

Because wsiadam is imperfective and fits a repeated, habitual action.

• wsiadam = I am getting on / I get on regularly or as a general habit
• wsiądę = perfective, usually I will get on once, as a completed future action

In this sentence, the meaning is habitual: whenever it rains, this is what I do. That is why wsiadam is the right choice.

After wsiadać / wsiąść with a vehicle like a bus, Polish normally uses do + genitive.

So:

• autobus = nominative
• do autobusu = genitive after do

That is why you get autobusu, not autobus.

Literally, the structure is close to get into the bus, though in English we usually say get on the bus.

Literally, it means I get on the bus or I board the bus.

In context, though, it clearly implies that the speaker chooses the bus as their way of getting to work, so the natural English meaning is close to I take the bus.

A very common Polish alternative would be:

Jadę autobusem = I go by bus / I take the bus

So the given sentence focuses a bit more on the action of boarding, while jadę autobusem focuses more on the means of transport.

Because after zamiast meaning instead of, Polish often uses the infinitive when the subject is the same as in the main clause.

So:

• zamiast iść = instead of going
• zamiast czekać = instead of waiting
• zamiast pracować = instead of working

In this sentence, the person who gets on the bus is also the person who would otherwise go on foot, so the infinitive works naturally.

This is a very common learner question.

• iść = to go, to walk, in one direction, on a specific trip
• chodzić = to go/walk habitually, repeatedly, or in general

Here, the sentence imagines each rainy-day commute as a concrete trip to work, so iść pieszo do pracy is natural.

A sentence with chodzić could also exist, but it would sound more like talking about the general habit of walking to work:

• zamiast chodzić pieszo do pracy = instead of walking to work regularly / as a habit

pieszo means on foot, and it is an adverb.

Polish uses pieszo with verbs of movement:

• iść pieszo = go on foot
• wracać pieszo = return on foot
• przyjść pieszo = come on foot

So in this sentence:

iść pieszo do pracy = go to work on foot

You do not use an adjective here; pieszo is the normal form.

Because Polish often uses do + genitive to show movement toward a destination.

Examples:

• iść do domu = go home
• iść do szkoły = go to school
• iść do pracy = go to work

Here:

• praca = work
• do pracy = to work

So iść pieszo do pracy means to walk to work.

Yes. In Polish, a subordinate clause introduced by kiedy is separated by a comma.

So this is correct:

Kiedy pada deszcz, wsiadam do autobusu...

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

Wsiadam do autobusu, kiedy pada deszcz.

Yes. Polish word order is flexible, and the sentence can be rearranged for emphasis.

The given version is neutral:

Kiedy pada deszcz, wsiadam do autobusu zamiast iść pieszo do pracy.

You could also say:

Kiedy pada deszcz, zamiast iść pieszo do pracy, wsiadam do autobusu.

The meaning stays basically the same, but the second version highlights the contrast with walking to work a bit more.