Rozstaw źrenic jest ważny.

Literally:

• rozstaw = spacing, distance apart, spread
• źrenic = of the pupils

So the phrase literally means the spacing of the pupils.

In natural English, this is usually called pupillary distance or interpupillary distance.

Because Polish is using the pattern:

• rozstaw czego? = the spacing of what?
• answer: źrenic = of the pupils

So źrenic is in the genitive plural.

The basic forms are:

• źrenica = pupil (singular)
• źrenice = pupils (plural, nominative)
• źrenic = of the pupils (genitive plural)

English usually keeps pupils unchanged in this kind of phrase, but Polish often uses the genitive after a noun like rozstaw.

Because the subject of the sentence is not źrenice. The subject is the whole phrase rozstaw źrenic.

The head word is rozstaw, and rozstaw is:

• singular
• masculine

So the adjective must match rozstaw, not źrenic:

• rozstaw → ważny

That is why you get:

• Rozstaw źrenic jest ważny.

Not:

• Rozstaw źrenic jest ważne.
• Rozstaw źrenic są ważne.

In English, it may feel like the sentence is about two pupils, but grammatically in Polish it is about one distance/spacing.

Jest means is. It is the 3rd person singular present tense of być = to be.

So the structure is:

• Rozstaw źrenic = subject
• jest = is
• ważny = important

Together: The pupillary distance is important.

Unlike in some other Slavic languages, Polish normally keeps to be in the present tense in ordinary sentences like this. Leaving out jest here would sound incomplete or very elliptical.

Yes. Polish word order is more flexible than English.

The given sentence:

• Rozstaw źrenic jest ważny.

is a normal, neutral order.

You could also say:

• Ważny jest rozstaw źrenic.

This puts more focus on ważny = important.

Both are grammatical. The difference is mainly emphasis, not basic meaning.

Because Polish does not have articles like a, an, and the.

So:

• rozstaw źrenic can mean the pupillary distance or just pupillary distance, depending on context.

English requires an article much more often; Polish does not.

A rough pronunciation guide is:

• rozstaw ≈ ROSS-stahf
• źrenic ≈ ZHREH-nyits
• jest ≈ yest
• ważny ≈ VAZH-nih

A few useful notes:

• ź in źrenic is a soft sound, softer than English zh
• final w in rozstaw sounds like f
• j in Polish sounds like English y
• ż in ważny sounds like the s in measure

Stress is normally on the second-to-last syllable of each word, so here that means:

• ROZ-staw
• ŹRE-nic
• jest
• WAŻ-ny

Yes, it is a standard expression, especially in optics and glasses fitting.

If you are talking about eyeglasses, lens alignment, or an optician’s measurement, rozstaw źrenic is a very natural phrase.

A more descriptive alternative would be:

• odległość między źrenicami = distance between the pupils

But rozstaw źrenic is shorter and more technical/standard in that context.

The whole phrase functions as the subject, so the main noun rozstaw is in the nominative singular.

Inside that phrase, źrenic is in the genitive plural.

So you can think of it like this:

• rozstaw = nominative singular
• źrenic = genitive plural

This kind of mixed structure is very common in Polish noun phrases. The main noun stays in the case required by the sentence, and the dependent noun changes according to the relationship being expressed.