Po pracy podgrzewam zupę w mikrofalówce.

Because po here requires the locative case.

• basic form: praca = work
• after po: po pracy = after work

This is a very common pattern in Polish:

• po obiedzie = after lunch
• po szkole = after school
• po filmie = after the movie

So in this sentence, po pracy means after work.

Because zupa is the direct object of the verb podgrzewam (I heat up), so it takes the accusative case.

• nominative: zupa = soup
• accusative: zupę = soup (as the thing being heated)

This is very common with feminine nouns ending in -a:

• mam kawę = I have coffee
• widzę kobietę = I see a woman
• jem pizzę = I eat pizza

So podgrzewam zupę literally means I am heating/heating up soup.

Podgrzewam means I heat up, I warm up, or I am reheating.

It comes from the verb podgrzewać, which is usually imperfective. That means it can describe:

• a habitual action: After work, I warm up soup
• what someone is doing in general / right now in context: I’m warming up soup

In everyday English, heat up or warm up is the most natural translation.

This is mainly about aspect.

• podgrzewać / podgrzewam = imperfective
• podgrzać / podgrzeję = perfective

In this sentence, podgrzewam sounds natural because it describes a routine or repeated action:

• Po pracy podgrzewam zupę. = After work, I warm up soup.

If you said:

• Po pracy podgrzeję zupę.

that would usually mean:

• After work, I will heat up the soup.

So podgrzewam fits a general habit, while podgrzeję points to one completed future action.

Because after w meaning in, Polish uses the locative case.

• basic form: mikrofalówka = microwave, microwave oven
• locative after w: w mikrofalówce = in the microwave

So:

• w kuchni = in the kitchen
• w szkole = at school / in school
• w lodówce = in the fridge
• w mikrofalówce = in the microwave

If you used mikrofalówkę, that would be the accusative, which would be used in a different structure, for example:

• kupuję mikrofalówkę = I’m buying a microwave

It most literally means in the microwave, but in natural English we often understand it as in the microwave / using the microwave.

So this Polish phrase tells you the location or method in a very natural way. It does not sound strange in Polish.

Compare:

• podgrzewam zupę w garnku = I heat the soup in a pot
• podgrzewam zupę w mikrofalówce = I heat the soup in the microwave

No, Polish word order is fairly flexible because the case endings show the grammatical roles.

The neutral order here is:

• Po pracy podgrzewam zupę w mikrofalówce.

But other orders are possible, depending on emphasis:

• Podgrzewam zupę w mikrofalówce po pracy.
• Zupę podgrzewam po pracy w mikrofalówce.

These all mean roughly the same thing, but the focus changes.

The original version sounds very natural because it starts with the time phrase po pracy (after work), which sets the scene first.

In Polish, subject pronouns are often omitted because the verb ending already shows who is doing the action.

• podgrzewam = I heat up
• the ending -am tells you it is I

So ja is usually unnecessary unless you want emphasis or contrast:

• Ja podgrzewam zupę, a on robi kanapki.
  I heat up the soup, and he makes sandwiches.

In the sentence you gave, leaving out ja is the most natural choice.

It usually means after work in the general everyday sense, not necessarily after the work.

So:

• Po pracy idę do domu. = After work, I go home.
• Po pracy podgrzewam zupę. = After work, I heat up soup.

If you wanted to refer to a specific piece of work or task, Polish would often express that more explicitly in another way.

A rough English-friendly guide:

• podgrzewam ≈ pod-GZHE-vam
• mikrofalówce ≈ mee-kro-fa-LOOF-tseh

A few useful notes:

• rz usually sounds like the zh in measure
• w sounds like English v
• ó sounds like oo
• ce at the end sounds like tseh

This is only an approximation, but it can help you get started.

In standard Polish, w mikrofalówce is the normal and natural phrase.

• mikrofalówka = microwave oven
• w mikrofalówce = in the microwave

You may hear shortened or colloquial expressions in speech, but for learners, w mikrofalówce is the best form to use.

Here are the main forms:

• po = after
• praca = work
• podgrzewać = to heat up / warm up
• zupa = soup
• w = in
• mikrofalówka = microwave oven

And here is how they appear in the sentence:

• po pracy → locative of praca
• podgrzewam → 1st person singular of podgrzewać
• zupę → accusative of zupa
• w mikrofalówce → locative of mikrofalówka