W piątek po pracy idziemy na spacer.

With days of the week, Polish usually uses w + accusative to mean on a given day:

• w poniedziałek
• we wtorek
• w środę
• w czwartek
• w piątek

So w piątek is the normal way to say on Friday.
W piątku is not correct here.

Usually w piątek means on Friday as a specific occasion, depending on context.

If you want to say every Friday, Polish more naturally uses:

• w piątki = on Fridays
• co piątek = every Friday

So:

• W piątek po pracy idziemy na spacer. = We’re going for a walk on Friday after work.
• W piątki po pracy idziemy na spacer. = On Fridays, after work, we go for a walk.

Here po means after, and it requires the locative case in this expression.

The noun is:

• praca = work

Its locative singular form is:

• pracy

So:

• po pracy = after work

This is a very common phrase in Polish.

Polish often leaves out subject pronouns because the verb ending already shows who is doing the action.

• idziemy = we go / we are going

So my is not necessary. You could say My idziemy na spacer, but that usually adds emphasis, like we are the ones going.

This is a very common Polish motion-verb issue.

• iść / idziemy = to go on foot, in one direction, on a particular occasion
• chodzić / chodzimy = to go on foot habitually, repeatedly, or in different directions

So here:

• idziemy na spacer = we’re going for a walk now / on this occasion
• chodzimy na spacery = we go for walks regularly

Because the sentence is about one planned walk, idziemy is the natural choice.

Yes. In Polish, the present tense of an imperfective verb is often used for a planned or arranged future action.

So idziemy literally looks like we are going, but in context it can mean:

• we’re going
• we’re going to go
• we’ll go

In this sentence, it sounds very natural as a planned future: On Friday after work, we’re going for a walk.

Yes, basically. Iść is the verb for going on foot in one direction.

That is different from English go, which is more general.

Compare:

• idziemy = we’re going on foot
• jedziemy = we’re going by vehicle

In this sentence, idziemy na spacer is perfect, because a spacer is a walk, so going on foot is exactly the idea.

Because na + accusative is used here for movement toward an activity or destination.

• iść na spacer = to go for a walk

But na spacerze is locative, and it means on a walk / while out for a walk, not movement toward it.

Compare:

• Idziemy na spacer. = We’re going for a walk.
• Jesteśmy na spacerze. = We’re on a walk.

It is accusative singular.

The noun is:

• spacer = walk

Because na here expresses direction/purpose, it takes the accusative.

For this noun, the accusative singular looks the same as the nominative singular:

• nominative: spacer
• accusative: spacer

So the form does not change, even though the case does.

Yes, you can. Both are natural:

• idziemy na spacer
• idziemy spacerować

But iść na spacer is an especially common fixed expression in Polish, much like go for a walk in English.

A small nuance:

• iść na spacer focuses on the outing itself
• iść spacerować focuses a bit more on the activity of walking

In everyday speech, both are fine.

Yes. Polish word order is fairly flexible, because endings carry a lot of the grammar.

The original sentence:

• W piątek po pracy idziemy na spacer.

You could also say:

• Po pracy w piątek idziemy na spacer.
• Idziemy na spacer w piątek po pracy.

These versions are all understandable, but the emphasis changes slightly. The original sounds very natural and neutral.

Polish sometimes uses we instead of w for easier pronunciation.

For example:

• we wtorek
• we Wrocławiu

This happens when w would be awkward before certain consonant clusters.

But w piątek is easy enough to pronounce, so normal Polish uses w, not we.

A practical approximation is PYON-tek, with the stress on the first syllable:

• pią-tek

A few useful pronunciation notes:

• pi sounds like a soft p plus y/ee
• ą before t is usually pronounced something like on
• Polish stress is usually on the second-to-last syllable, so here it falls on pią

So piątek sounds roughly like PYON-tek, though the exact Polish sounds are a bit different from English.

Not really in the English sense. Polish does not use articles like the or a, so po pracy simply means after work.

That is the normal idiomatic way to say it. English speakers often want to insert an article mentally, but Polish just does not work that way.