Po práci se zastavím na poště.

Because po in the sense of after takes the locative case in Czech.

• basic form: práce = work
• locative singular: práci
• so: po práci = after work

This is a very common pattern:

• po škole = after school
• po obědě = after lunch
• po večeři = after dinner

So in this sentence, po práci is simply the correct case form after po.

Because the verb here is zastavit se, not just zastavit.

These are different:

• zastavit = to stop something
• zastavit se = to stop, stop by, drop in

So:

• Zastavím auto. = I will stop the car.
• Zastavím se na poště. = I will stop by at the post office.

The se is not really translated as myself here. It is just part of the verb’s normal form.

In this sentence, it means future.

That is because zastavit se is a perfective verb. In Czech, the present-tense form of a perfective verb usually has future meaning.

So:

• zastavím se = I will stop by

Even though it looks like a present form, it is understood as future.

Here it means something like:

• to stop by
• to drop in
• to make a stop

So Po práci se zastavím na poště means that after work, the speaker plans to make a stop at the post office.

It does not necessarily mean coming to a physical halt in a dramatic sense; it is the ordinary idea of making a brief stop somewhere.

Because Czech uses na poště as the normal idiomatic expression for at the post office.

With many public places and institutions, Czech uses na where English speakers might expect in or at. This is something you often just have to learn as part of the noun phrase.

So:

• na poště = at the post office
• na univerzitě = at the university
• na nádraží = at the station

Using v poště would not sound natural here.

Because after na in this phrase, Czech uses the locative case.

• dictionary form: pošta
• locative singular: poště
• phrase: na poště

So both práci and poště are case forms required by the words before them:

• po práci → locative after po
• na poště → locative after na

This is a very common learner question.

• na poštu usually means to the post office as a destination
• na poště means at the post office

With the verb zastavit se, Czech normally uses the place where you stop as a location, so na poště is the natural form:

• Zastavím se na poště. = I’ll stop by at the post office.

Compare:

• Jdu na poštu. = I’m going to the post office.
• Jsem na poště. = I’m at the post office.

So this sentence uses the standard idiomatic pattern with zastavit se.

Yes, Czech word order is flexible, but the original sentence is a very natural neutral version:

• Po práci se zastavím na poště.

You could also say:

• Na poště se zastavím po práci.
• Se zastavím na poště po práci. — much less natural in ordinary speech

What matters is that se is a clitic, so it usually goes in the second position of the sentence or clause. That is why Po práci se zastavím... sounds right, while Po práci zastavím se... sounds wrong.

It sounds most naturally like a single planned future action:

• After work, I’ll stop by the post office.

Because zastavím se is perfective, it usually refers to one completed event.

If you wanted to describe a habit, Czech would more naturally use an imperfective form, for example:

• Po práci se zastavuji na poště. = I stop by the post office after work. / I usually stop by the post office after work.

So the original sentence sounds like a specific plan, not a routine.

Here pošta means post office.

But depending on context, pošta can also mean:

• mail
• post
• postal service

In na poště, the meaning is clearly at the post office.

Two parts may feel tricky for English speakers:

• práci contains ř, the famous Czech sound
• poště contains šť

A rough guide:

• práci ≈ praa-tsi, but with ř pronounced as the special Czech sound, not a normal English r
• poště ≈ posh-tye

Also note:

• á is long
• š sounds like sh
• ť is a soft t

If you cannot say ř yet, a rough temporary approximation is better than nothing, but it is worth practicing because it is an important Czech sound.