Na tym dokumencie jest ważna data, więc sprawdzam ją dwa razy.

In Polish, na is often used for things that are physically on the surface of something, while w usually means in/inside.

So:

• na dokumencie = on the document
• w dokumencie = in the document

With a date printed or written on a document, Polish normally uses na, because the date appears on it.

Because the preposition na here requires the locative case.

The basic form is:

• ten dokument = this document

But after na in the meaning on, the noun goes into the locative:

• na tym dokumencie = on this document

Breakdown:

• ten → tym
• dokument → dokumencie

This is a normal case change in Polish.

Because adjectives in Polish must agree with the noun in gender, number, and case.

Here:

• data is a feminine singular noun
• so the adjective must also be feminine singular

That gives:

• ważna data = an important date

Compare:

• ważny dokument = an important document (masculine)
• ważna data = an important date (feminine)
• ważne miejsce = an important place (neuter)

Jest means is.

In this sentence:

• Na tym dokumencie jest ważna data
  = There is an important date on this document

Polish often uses jest to express existence, similar to English there is.

You could sometimes hear shorter, more conversational wording without jest, but in this sentence jest is the natural standard form.

Because data is the subject of the clause jest ważna data.

Subjects are normally in the nominative case, which is the dictionary form.

So:

• data = nominative singular
• ważna also matches it in nominative singular feminine

That is why you see ważna data, not a different case form.

Więc means so, therefore, or thus.

It links the two parts of the sentence:

• Na tym dokumencie jest ważna data = there is an important date on this document
• więc sprawdzam ją dwa razy = so I check it twice

It works much like English so in the middle of a sentence.

Sprawdzam is the 1st person singular present tense of sprawdzać, meaning I check / I am checking.

So:

• sprawdzam = I check or I am checking

Polish present tense often covers both the simple present and the present continuous meanings that English separates.

In context, sprawdzam ją dwa razy means something like:

• I check it twice
• or I’m checking it twice

depending on the situation.

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

• sprawdzać = imperfective
• sprawdzić = perfective

Imperfective is used for repeated, habitual, ongoing, or non-completed actions.

Since the sentence says dwa razy (twice), the idea is that the speaker checks it repeatedly, so sprawdzam from sprawdzać is very natural.

A perfective form like sprawdzę would mean more like I will check / I will have checked, and it would not fit this sentence in the same way.

Ją means her or it, depending on context. Here it means it, referring to data.

Why ją?

Because:

1. data is a feminine noun
2. it is the direct object of sprawdzam
3. direct objects often go into the accusative case

For feminine singular nouns/pronouns, the accusative pronoun is:

• ją = her/it

So:

• sprawdzam ją = I check it

Here ją refers specifically to ważna data.

You could say sprawdzam dwa razy, but then the object it is no longer explicit.

• sprawdzam ją dwa razy = I check it twice
• sprawdzam dwa razy = I check twice

The second version is possible, but it sounds less specific. Since the sentence clearly refers back to data, using ją is natural and clear.

Because dwa razy is a fixed expression meaning twice, and the numeral is agreeing with razy, not with data.

The key word here is:

• raz = time / occurrence

In counting expressions:

• raz = once
• dwa razy = twice
• trzy razy = three times

So the gender of data does not matter here. The phrase is about the number of times the action happens.

Yes, Polish word order is more flexible than English, but different orders can sound more natural in different contexts.

The given sentence:

• Na tym dokumencie jest ważna data, więc sprawdzam ją dwa razy.

is neutral and natural.

You might also hear:

• Jest ważna data na tym dokumencie, więc sprawdzam ją dwa razy.

This is understandable, but the original version sounds smoother because it starts with the location on this document.

Polish often changes word order for emphasis, style, or information structure, not because the grammar changes.

Here are the main dictionary forms:

• na = on
• ten = this
• dokument = document
• być → jest = to be → is
• ważny = important
• data = date
• więc = so
• sprawdzać → sprawdzam = to check → I check
• ona / ją = she/it / her/it
• dwa = two
• raz = time, occasion

This is useful because Polish sentences often contain inflected forms that look different from the dictionary entry.

Usually, yes, when you mean a calendar date.

For example:

• Jaka jest data? = What is the date?
• ważna data = an important date

Just remember that data in Polish is a normal feminine noun, so it behaves grammatically like other feminine nouns. That is why you later get ją when referring back to it.