Po pracy kupuję nie tylko jogurt, ale jeszcze chleb i mleko.

Because the preposition po in the meaning after requires the locative case.

• Base form: praca = work
• Locative singular: pracy

So po pracy means after work.
This is one of those forms where the ending changes because of the preposition.

Kupuję is the 1st person singular present tense form of kupować (to buy, imperfective).

So it means:

• I buy
• I am buying

depending on context.

Because the verb is imperfective, it often suggests a repeated or habitual action. In this sentence, with po pracy, it very naturally sounds like a routine: After work, I buy...

A useful contrast:

• kupuję = I buy / I’m buying
• kupię = I will buy / I’ll buy once

Polish often leaves out subject pronouns when they are not needed.

The ending -ę in kupuję already tells you the subject is I. So:

• kupuję = I buy / I am buying
• ja kupuję = I buy, but with extra emphasis on I

Polish uses ja only when it is important to stress the subject, contrast it with someone else, or make the sentence especially clear.

This is the pattern not only ..., but also ...

So:

• nie tylko jogurt = not only yogurt
• ale jeszcze chleb i mleko = but also bread and milk

The sentence is building contrast and addition:

• first item: jogurt
• extra items: chleb i mleko

This is very similar to English not only ..., but also ...

They are the direct objects of the verb kupuję, so they are in the accusative case.

But in Polish, the accusative is often identical to the base form for these kinds of nouns:

• jogurt — masculine inanimate
• chleb — masculine inanimate
• mleko — neuter

For both masculine inanimate singular and neuter singular, the accusative usually looks the same as the nominative.

So:

• jogurt stays jogurt
• chleb stays chleb
• mleko stays mleko

A useful comparison:

• mam kota = I have a cat
  Here kot changes to kota, because masculine animate nouns behave differently.

It is understandable, but many speakers would more naturally say:

• nie tylko jogurt, ale też chleb i mleko
• nie tylko jogurt, ale także chleb i mleko
• nie tylko jogurt, ale również chleb i mleko

Using jeszcze gives a sense of and in addition / and on top of that. It is not wrong, but ale też / ale także / ale również is often the more standard or expected pattern in neutral Polish.

Yes. In standard Polish spelling, you normally put a comma before ale.

So:

• Po pracy kupuję nie tylko jogurt, ale jeszcze chleb i mleko.

That comma helps mark the contrast between the two parts of the sentence.

Yes. Polish word order is much more flexible than English word order.

This sentence begins with Po pracy, which puts the time expression first. That sounds natural and gives the sentence a clear setting: After work...

You could also say:

• Kupuję po pracy nie tylko jogurt, ale jeszcze chleb i mleko.
• Nie tylko jogurt kupuję po pracy, ale jeszcze chleb i mleko.

These versions are grammatically possible, but they change the emphasis. The original sentence is a very natural, neutral way to say it.

Polish does not have articles.

So a noun like jogurt can mean:

• yogurt
• a yogurt
• the yogurt

depending on context.

The same is true for chleb and mleko. English requires articles much more often, but Polish usually leaves that information to context.

Also, chleb and mleko are often understood as general or uncountable substances, like bread and milk in English.

It can mean either one, depending on context.

By itself, po pracy just means after work. The tense and the situation tell you whether it is:

• a routine: After work, I buy...
• a current plan: After work, I’m buying...

Since kupuję is present tense and often works for habits, many learners will first read this as a regular habit or routine.

A rough English-friendly guide is:

• kupuję ≈ koo-POO-yeh

A few important points:

• Stress is usually on the second-to-last syllable in Polish, so here it falls on pu: ku-PU-ję
• The j sounds like English y
• Final ę is often pronounced less strongly nasal than learners expect, and in everyday speech it may sound close to e with only slight nasal coloring

So if you pronounce it roughly as ku-POO-ye, you will be understood, even if the exact Polish sound takes practice.