Po práci půjdu na poštu s malým balíkem.

In this sentence, po means after because it is followed by a noun in the locative case:

• po práci = after work

This is a very common pattern in Czech:

• po škole = after school
• po obědě = after lunch
• po práci = after work

So po does not always mean along or on. Its meaning depends on the case and the context.

Because po here requires the locative case.

The noun práce changes like this in the singular:

• práce = nominative
• práci = locative

So:

• po práci = after work

This is one of the things English speakers have to get used to in Czech: prepositions often force a specific case.

Půjdu is future tense and means I will go.

So:

• jdu = I am going / I go
• půjdu = I will go

In this sentence:

• Po práci půjdu na poštu... = After work, I will go to the post office...

Many Czech verbs form the future with budu + infinitive, but jít is one of the important exceptions.

For jít:

• jdu = I am going
• půjdu = I will go

You do not say budu jít in normal Czech for this meaning.

This is worth memorizing as a special future form of jít.

Na poštu is the normal Czech way to say to the post office.

Czech often uses na with places understood as institutions, destinations, or functional places:

• na poštu = to the post office
• na nádraží = to the station
• na policii = to the police station / police

Using do would usually focus more on going into a place, while na poštu is the standard idiomatic expression for going there as your destination.

So for learners, the safest choice is:

• jít na poštu = to go to the post office

Because na with movement usually takes the accusative case.

You are going to the post office, so Czech treats pošta as the destination:

• na poštu

Here the noun pošta changes from:

• pošta = nominative
• poštu = accusative

Compare:

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

That contrast is very important:

• na + accusative = motion toward
• na + locative = location

Because the preposition s meaning with takes the instrumental case.

So:

• s balíkem = with a package
• s malým balíkem = with a small package

Both the adjective and the noun have to match the instrumental case:

• malý → malým
• balík → balíkem

The endings show it:

• masculine/inanimate adjective in instrumental singular often ends in -ým
• masculine/inanimate noun in instrumental singular often ends in -em

So:

• malý → malým
• balík → balíkem

This is a very typical Czech pattern, and you will see it often after s:

• s dobrým kamarádem = with a good friend
• s novým autem = with a new car
• s malým balíkem = with a small package

Czech does not have articles like English a and the.

So:

• pošta can mean post office or the post office
• balík can mean a package, the package, or just package

The exact meaning comes from context.

That means na poštu s malým balíkem naturally covers what English would express as:

• to the post office with a small package

Because Czech verbs usually show the person clearly enough on their own.

• půjdu already means I will go

So the pronoun já is usually unnecessary unless you want emphasis or contrast.

Compare:

• Po práci půjdu na poštu. = After work, I’ll go to the post office.
• Po práci půjdu na poštu já. = I will go to the post office after work.
  (emphasized, maybe not someone else)

No, Czech word order is fairly flexible, though some orders sound more neutral than others.

The original sentence:

• Po práci půjdu na poštu s malým balíkem.

is a very natural, neutral way to say it.

You could also say:

• Půjdu po práci na poštu s malým balíkem.
• Na poštu půjdu po práci s malým balíkem.

These are all understandable, but they shift emphasis a little.

A good rule for learners is:

• put time expressions like po práci near the beginning
• keep the main verb early
• put extra details like s malým balíkem later

Yes, balík usually means a package, parcel, or box/package to be sent or carried.

In this sentence, because of na poštu, it strongly suggests a package connected with mailing.

So:

• balík = package / parcel

A learner might also see balíček, which is a smaller package or sometimes just a more diminutive, friendlier form.

Yes, that is a natural interpretation of the overall meaning, but grammatically the Czech sentence mainly says:

• After work, I will go to the post office with a small package.

The phrase s malým balíkem tells you what you are bringing or carrying with you. It does not explicitly say whether you are mailing it, picking something up, or just taking it there. That part is understood from context.

So the sentence gives the destination and the accompanying object, but not the exact purpose.