Po pracy mam już zrobione zakupy, ale nie chcę już usuwać chwastów.

Because po in the sense of after takes the locative case here. The base noun is praca, and its locative singular form is pracy.

So:

• praca = work
• po pracy = after work

This is a very common pattern:

• po szkole = after school
• po obiedzie = after lunch
• po spotkaniu = after the meeting

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

Here:

• mam = I have
• chcę = I want

So ja is not necessary. If you added ja, it would sound more emphatic, like:

• Ja nie chcę już usuwać chwastów = I don’t want to remove weeds anymore

That might be used for contrast, for example if someone else does want to do it.

Here już means already.

So mam już zrobione zakupy means that the shopping is already done by this point.

It often suggests that something has been completed earlier than expected, or at least that it is now finished and out of the way.

In Polish, zakupy is normally used in the plural to mean shopping or groceries/shopping errands.

So:

• robić zakupy = to do the shopping
• zrobić zakupy = to do the shopping / complete the shopping

The singular zakup usually means a purchase or one bought item, and can sound more formal or technical.

So in everyday Polish, zakupy is the natural choice here.

Both are possible, but they focus on slightly different things.

• Zrobiłem zakupy = I did the shopping
  This focuses on the action.

• Mam zrobione zakupy = I have the shopping done
  This focuses on the resulting state: the shopping is now taken care of.

This mieć + participle pattern is very common when talking about completed tasks:

• Mam posprzątany pokój = I have the room cleaned
• Mam napisany raport = I have the report written

So here the speaker is emphasizing that the shopping is already finished.

Zrobione is a participle/adjective-like form from zrobić. In this structure, it agrees with the noun it describes.

Here it matches zakupy, which is plural, so we get zrobione zakupy.

Compare:

• mam kupiony bilet = I have the ticket bought
• mam kupioną książkę = I have the book bought
• mam kupione buty = I have the shoes bought

So the form changes depending on the gender and number of the noun.

Not quite. Już is the same word, but its meaning changes a little depending on context.

First już

In mam już zrobione zakupy, it means already.

Second już

In nie chcę już usuwać chwastów, it means something more like anymore or any longer.

So:

• już in a positive clause often = already
• już with negation often = anymore / no longer

That is why the two już words feel different, even though they are the same word.

This is about aspect.

• usuwać = imperfective
• usunąć = perfective

The imperfective usuwać focuses on the activity as a process, something ongoing or repeated. That works well with a chore like removing weeds.

So nie chcę już usuwać chwastów suggests:

• I don’t want to keep doing this activity anymore
• I don’t want to do any more weeding

If you used usunąć, the focus would be more on finishing the job completely:

• Nie chcę usunąć chwastów = I don’t want to remove the weeds

That is possible, but it sounds more like refusal to complete a one-time result, rather than not wanting to continue the activity.

Because of negation.

In standard Polish, when a direct object is governed by a negated verb, it often changes to the genitive.

Compare:

• Chcę usuwać chwasty = I want to remove weeds
• Nie chcę usuwać chwastów = I don’t want to remove weeds

Here:

• chwasty = accusative/nominative plural
• chwastów = genitive plural

This is a very important Polish pattern and is especially common after nie.

Because in Polish, ale normally introduces a new clause, and a comma is required before it.

So:

• mam już zrobione zakupy
• ale nie chcę już usuwać chwastów

These are two coordinated clauses, so the comma is standard and necessary.

This is one of those punctuation rules that is stricter in Polish than many learners expect.

Yes. Polish word order is quite flexible, and changing it usually changes emphasis, not the basic meaning.

For example:

• Po pracy mam już zrobione zakupy...
  This starts with the time frame: after work

• Mam już zrobione zakupy po pracy...
  Much less natural in this context

• Już nie chcę usuwać chwastów
  Strongly emphasizes no longer

• Nie chcę już usuwać chwastów
  Emphasizes that I don’t want to continue doing it anymore

So the original order is natural and well balanced.

It is correct and completely understandable, but it can sound a bit neutral or formal.

In everyday speech, Polish speakers might also say:

• wyrywać chwasty = pull weeds
• plewić = weed

So depending on style, you might also hear:

• Nie chcę już wyrywać chwastów
• Nie chcę już plewić

But usuwać chwastów is still a good, grammatical choice.