To jest mój obowiązek, więc zrobię to dziś.

The two to words do different jobs.

• The first To means this/that/it in a general identifying sense:
  To jest mój obowiązek = This/That is my duty / It is my duty
• The second to is the object of zrobię:
  zrobię to = I will do it

So even though both are spelled to, they are functioning differently in the sentence.

After to jest, Polish very often uses the nominative case when identifying something.

So:

• To jest mój obowiązek = This is my duty

Here:

• mój is nominative masculine singular
• obowiązek is nominative masculine singular

You may learn that after być (to be), Polish sometimes uses the instrumental case, especially in sentences like:

• Jestem nauczycielem = I am a teacher

But with the structure to jest..., nominative is very common and natural:

• To jest problem
• To jest mój obowiązek
• To jest dobra okazja

Mój is in the nominative masculine singular form because it agrees with obowiązek.

Breakdown:

• obowiązek = a masculine singular noun
• mój must match it in gender, number, and case

So:

• nominative masculine singular: mój obowiązek
• instrumental masculine singular: moim obowiązkiem
• genitive masculine singular: mojego obowiązku

In this sentence, the correct form is mój obowiązek.

Polish usually does not need subject pronouns if the verb ending already shows who is doing the action.

• zrobię means I will do
• the ending -ę tells you the subject is I

So:

• zrobię = I will do
• ja zrobię = I will do with extra emphasis

You would add ja only if you want contrast or stress, for example:

• Ja zrobię to dziś, a ty jutro.
  I will do it today, and you tomorrow.

In your sentence, leaving out ja is normal and natural.

This is about aspect, which is very important in Polish.

• robić = imperfective = to do, to be doing, to do repeatedly
• zrobić = perfective = to do, to complete, to get done

The form zrobię is the future tense of the perfective verb zrobić, and it means:

• I will do it
• often with the sense I will get it done / complete it

That fits well here because the speaker is talking about a duty and a completed action:

• więc zrobię to dziś = so I’ll do it today

If you said będę robić to dziś, it would sound more like:

• I will be doing it today

That focuses more on the process, not the completion.

It is future tense.

In Polish, perfective verbs do not normally have a present tense meaning. Their so-called present forms actually refer to the future.

So:

• robię = I am doing / I do (imperfective, true present)
• zrobię = I will do / I will get done (perfective, future)

This is a key pattern in Polish:

• piszę = I am writing
• napiszę = I will write
• jem = I am eating
• zjem = I will eat up / finish eating

Więc means so, therefore, or thus.

In this sentence:

• To jest mój obowiązek, więc zrobię to dziś.
• It is my duty, so I will do it today.

The comma is used because więc connects two clauses:

• To jest mój obowiązek
• zrobię to dziś

This is normal Polish punctuation.

A close alternative is dlatego, but it works a little differently:

• To jest mój obowiązek, więc zrobię to dziś.
• To jest mój obowiązek, dlatego zrobię to dziś.

Both are possible, but więc is very common in everyday speech.

Yes, Polish word order is more flexible than English word order, because case endings and verb forms carry a lot of information.

The version in your sentence:

• zrobię to dziś

is a neutral, natural way to say:

• I’ll do it today

Other possible orders include:

• Dziś to zrobię = Today I’ll do it
  (more emphasis on today)
• To zrobię dziś = This/it I’ll do today
  (more emphasis on it)
• Zrobię dziś to
  possible, but less natural in many contexts

So the word order can change, but the emphasis changes too. The original version is a very standard choice.

Putting dziś at the end is very natural, especially when it adds the final piece of information.

• zrobię to dziś = I’ll do it today

Polish can place time expressions in different positions:

• Dziś zrobię to
• Zrobię to dziś
• To dziś zrobię

All are possible, but they sound slightly different in emphasis.

In your sentence, dziś at the end sounds neutral and smooth. It puts a mild focus on today as the planned time.

Both mean today.

• dziś = shorter
• dzisiaj = slightly longer, also very common

In most situations, they are interchangeable:

• Zrobię to dziś
• Zrobię to dzisiaj

Both are correct and natural.

Very roughly:

• dziś can feel a little more compact or stylistically neat
• dzisiaj can feel a little more conversational in some contexts

But for a learner, the safest rule is: both mean today, and both are common.

Usually no, not in this sentence.

• To jest mój obowiązek is the normal structure
• Jest mój obowiązek sounds incomplete or unnatural in standard usage

The initial To helps create the identifying pattern This/That is...

Compare:

• To jest mój obowiązek. = This/That is my duty.
• To jest problem. = This/That is a problem.
• To jest dobre rozwiązanie. = This/That is a good solution.

So in this sentence, the first To is important.

Obowiązek means duty, obligation, or responsibility, depending on context.

In this sentence, mój obowiązek means:

• my duty
• possibly my responsibility

It is a masculine noun, which is why it goes with mój.

Some related examples:

• Mam obowiązek to zrobić. = I have a duty/an obligation to do it.
• To nie jest mój obowiązek. = That is not my duty.
• Obowiązki domowe = household duties/chores

So here, the speaker is saying that because it is their duty, they will do it today.

A rough pronunciation guide is:

• zrobię ≈ zro-byeh

A few details:

• rz in zrobię is pronounced like the Polish sound written ż
• bię is pronounced with a soft b sound before e
• the final ę at the end is often pronounced less strongly than learners expect; in normal speech it may sound close to e with some nasal quality depending on context

A simple learner-friendly approximation is:

• ZROH-byeh

The stress is on the second-to-last syllable:

• zro-BIĘ

Yes, you could.

• zrobię to dziś
• to zrobię dziś

Both are grammatical.

The difference is emphasis:

• zrobię to dziś = more neutral
• to zrobię dziś = slightly more emphasis on it/that

So if you want a plain, unmarked sentence, the original version is better. If you want to stress that particular thing, then to zrobię dziś works well.