Po pracy mam jeszcze jedną ważną rzecz do zrobienia.

Because po requires the locative case when it means after in a time expression.

• praca = work
• po pracy = after work

So pracy is the locative singular form of praca.

This is a very common pattern:

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

So in this sentence, po pracy simply means after work.

Mam is the 1st person singular form of mieć in the present tense.

• mieć = to have
• mam = I have

So mam jeszcze jedną ważną rzecz do zrobienia literally starts as I have still/one more important thing to do.

In Polish, mieć coś do zrobienia is a very common way to say that you have a task waiting for you.

Here jeszcze means something like still, yet, or one more depending on how you look at the whole phrase.

In mam jeszcze jedną ważną rzecz do zrobienia, it gives the idea that there is an additional task remaining.

So it suggests:

• I still have...
• I have yet another...
• I have one more...

Very often jeszcze adds the idea of continuation or addition:

• Jeszcze pracuję. = I’m still working.
• Chcę jeszcze kawę. = I want some more coffee.

This phrase is in the accusative case, because it is the direct object of mam.

The base forms are:

• jedna = one
• ważna = important
• rzecz = thing

In the accusative singular feminine, the modifiers change:

• jedna → jedną
• ważna → ważną
• rzecz stays rzecz

So:

• jedną ważną rzecz = one important thing

This is a useful pattern to notice: with feminine nouns, adjectives and words like jedna often change to -ą in the accusative singular.

Because some feminine nouns do not visibly change between the nominative and accusative singular.

So:

• ta rzecz = this thing
• widzę tę rzecz = I see this thing

The noun rzecz stays the same in form, but the words around it show the case:

• jedna ważna rzecz = nominative
• jedną ważną rzecz = accusative

This can feel strange at first for an English speaker, but it is completely normal in Polish.

Because Polish often uses the pattern mieć coś do zrobienia rather than an infinitive after mieć.

So instead of saying something like I have something to do with an infinitive structure, Polish uses:

• mieć
  • object + do
    • verbal noun

Here:

• zrobienie = doing / completion of doing
• do zrobienia = to be done

So:

• mam rzecz do zrobienia = I have a thing to do

This is one of the most natural Polish ways to express pending tasks.

Zrobienia is the genitive singular form of the verbal noun zrobienie, which comes from the verb zrobić.

• zrobić = to do / to make / to complete
• zrobienie = doing, making, completion

After do, Polish normally uses the genitive, so:

• do zrobienia

You do not need to think of it as a separate English-style word. It is best learned as part of the whole pattern:

• do zrobienia = to do
• do napisania = to write
• do przeczytania = to read

Yes, it is a very common and highly natural pattern.

You can use it with many nouns and many verbs:

• Mam pracę do skończenia. = I have work to finish.
• Mam maila do napisania. = I have an email to write.
• Mam kilka rzeczy do załatwienia. = I have several things to take care of.

So in your sentence, mam jeszcze jedną ważną rzecz do zrobienia is a standard, idiomatic Polish structure.

Yes, you could leave it out, but the meaning would shift slightly.

Compare:

• Mam jeszcze jedną ważną rzecz do zrobienia.
• Mam jeszcze ważną rzecz do zrobienia.

The version with jedną emphasizes one more specific thing. It sounds very natural when you want to count or highlight that there is exactly one remaining task.

Without jedną, the sentence is still possible, but it sounds less precise and slightly less natural in this context. The version with jedną is better if you mean one more important thing.

Because in Polish, adjectives usually come before the noun.

So:

• ważna rzecz = an important thing
• ciekawa książka = an interesting book
• duży dom = a big house

That is the neutral, standard order. Polish word order is flexible, but ważną rzecz is the normal choice here.

Yes, Polish word order is flexible, but different orders sound more or less natural depending on emphasis.

The original sentence:

• Po pracy mam jeszcze jedną ważną rzecz do zrobienia.

This is very natural because it starts with the time expression po pracy and then gives the main message.

You could also say:

• Mam jeszcze jedną ważną rzecz do zrobienia po pracy.

This is also possible, but it may sound a bit more like you are emphasizing after work at the end, or that the task is connected specifically with that time slot.

So the original order is a very natural neutral choice.

Yes. That is also correct, but it means something slightly different.

• jedną ważną rzecz = one important thing
• coś ważnego = something important

So:

• Po pracy mam jeszcze jedną ważną rzecz do zrobienia emphasizes one specific remaining task
• Po pracy mam jeszcze coś ważnego do zrobienia is a bit less specific and means I still have something important to do after work

Both are natural, but the original sentence is more precise.