Normalnie wracam autobusem, ale dziś idę pieszo.

Here normalnie means normally / usually.

So Normalnie wracam autobusem means something like I normally go back by bus or I usually take the bus back.

A small note: in casual Polish, normalnie can sometimes also mean seriously, literally, or can you believe it, depending on tone. But in this sentence it simply means usually.

Polish often leaves out subject pronouns when they are clear from the verb ending.

• wracam = I return / I’m returning
• idę = I go / I’m going

So ja is not necessary. You could say Ja normalnie wracam autobusem..., but that would add emphasis, a bit like I normally go back by bus...

Wracam comes from wracać, which means to return / to come back / to go back.

So the sentence is specifically about the speaker’s return journey. That is why Polish uses wracam.

• wracam autobusem = I return by bus
• jadę autobusem = I go / travel by bus

In natural English, you might translate the whole idea more freely as I usually take the bus, but today I’m walking, even though Polish literally says I normally return by bus.

Because Polish uses the present tense for habitual actions, just like English can use the simple present for habits.

• Normalnie wracam autobusem = I usually go back by bus
• W soboty pracuję = I work on Saturdays

So the present tense in Polish can mean:

• something happening now, or
• something that happens regularly

The adverb normalnie makes it clear that this is a habit.

Because autobusem is the instrumental case of autobus.

After many verbs, Polish uses the instrumental to express means of transport:

• autobusem = by bus
• pociągiem = by train
• samochodem = by car
• tramwajem = by tram

So:

• wracam autobusem = I return by bus

This is a very common pattern in Polish.

This is a very common and important distinction.

• iść / idę = going in one specific direction, on one particular trip, often right now
• chodzić / chodzę = going/walking habitually, repeatedly, or in no particular one-way direction

In this sentence, dziś idę pieszo means today I’m going on foot / today I’m walking for this specific trip.

Compare:

• Dziś idę pieszo. = Today I’m walking (this time).
• Zwykle chodzę pieszo. = I usually go on foot / I usually walk.

Pieszo is an adverb meaning on foot.

So:

• iść pieszo = to go on foot
• wracać pieszo = to return on foot

It is not a noun form and it does not change for case here. It is just a fixed adverb.

English often uses walk where Polish uses go on foot:

• Idę pieszo. = I’m walking / I’m going on foot.

They mean the same thing: today.

• dziś
• dzisiaj

Both are very common and natural. Dziś is simply shorter. In this sentence, dziś sounds perfectly normal.

It can mean either, depending on context.

Polish does not have a separate tense exactly like the English present continuous. So idę can be:

• I go
• I am going

In this sentence, because of dziś and the contrast with the usual habit, dziś idę pieszo is most naturally understood as:

• today I’m going on foot or
• today I’m walking

Yes. Polish word order is fairly flexible, though some orders sound more natural in a given context.

The original sentence:

Normalnie wracam autobusem, ale dziś idę pieszo.

puts a nice contrast between:

• normalnie = normally
• dziś = today

You could also say:

• Dziś idę pieszo, ale normalnie wracam autobusem.
• Autobusem normalnie wracam, ale dziś idę pieszo.
  (more marked, with special emphasis)

So the original version is natural and balanced, but other orders are possible if you want different emphasis.

Because ale means but, and in Polish it normally introduces a new clause, so it takes a comma.

• Normalnie wracam autobusem, ale dziś idę pieszo.

This is standard Polish punctuation.