Kiedy duża wskazówka jest na dwunastej, mała wskazówka pokazuje godzinę.

Because they agree with feminine nouns.

• wskazówka is a feminine noun, so its adjectives are:
  • duża wskazówka
  • mała wskazówka
• In na dwunastej, the word dwunastej refers to an implied feminine noun such as godzinie or pozycji. So it also appears in a feminine form.

This kind of agreement is very important in Polish: adjectives usually match the noun in gender, number, and case.

In this sentence, wskazówka means a hand of a clock.

So:

• duża wskazówka = the big hand = the minute hand
• mała wskazówka = the small hand = the hour hand

Outside clock contexts, wskazówka can also mean:

• a pointer
• a needle (for example on a gauge)
• a hint or tip in some contexts

But here it clearly means a clock hand.

This is a Polish idiomatic way to talk about the position of a clock hand.

• na dwunastej literally means something like at the twelve position
• It uses na
  • the locative case

The full idea is something like:

• na dwunastej godzinie
• na pozycji dwunastej

In everyday speech, the noun is often omitted, leaving just na dwunastej.

So the sentence is not using twelve as a plain number in the English way. It is treating dwunasta as the twelve o’clock position on the clock face.

Because after na in this meaning, Polish uses the locative case.

• basic form: dwunasta
• locative form: dwunastej

So:

• dwunasta = twelve / the twelfth / twelve o’clock position (base form)
• na dwunastej = at the twelve position

This is the same kind of case change you see in many Polish prepositional phrases.

Jest na dwunastej is a normal way to say is at the twelve position.

• jest simply means is
• It describes the hand’s current position

You may also hear other verbs in related contexts, but jest is very straightforward and natural here.

So the phrase means:

• the big hand is on twelve
• more literally: the big hand is at the twelve position

Because pokazuje takes a direct object, and direct objects usually go in the accusative case.

• infinitive: pokazywać = to show
• pokazuje godzinę = shows the hour

Here:

• nominative: godzina
• accusative: godzinę

So the sentence is structured like:

• mała wskazówka = subject
• pokazuje = verb
• godzinę = object

Here it means the hour in the sense of which hour it is.

It does not mean a duration of sixty minutes here.

So mała wskazówka pokazuje godzinę means:

• the small hand indicates the hour
• the small hand tells you which hour it is

This is a common thing for learners to notice, because English distinguishes more clearly between:

• an hour = duration
• the hour = clock reading / hour number

Polish godzina / godzinę can work in both types of meaning depending on context.

Yes, but it would be a different structure.

For example:

• Kiedy duża wskazówka jest na dwunastej, mała wskazówka pokazuje, która jest godzina.

That means something like:

• When the big hand is on twelve, the small hand shows what hour it is.

The version in your sentence, pokazuje godzinę, is shorter and more instructional. It sounds like the kind of sentence you might see in a textbook explaining how to read an analog clock.

Here Kiedy means when.

In this sentence, it introduces a time clause:

• Kiedy duża wskazówka jest na dwunastej...
• When the big hand is on twelve...

Depending on context, Polish kiedy can sometimes feel like when or whenever, just as English can. In this sentence, it is giving a general rule, so in English you might also understand it as:

• Whenever the big hand is on twelve, the small hand shows the hour.

Yes. Gdy would also be possible here.

• Kiedy duża wskazówka jest na dwunastej...
• Gdy duża wskazówka jest na dwunastej...

Both mean when.

Very broadly:

• kiedy is extremely common and neutral
• gdy can sound a little more formal or literary in some contexts

For a learner, kiedy is usually the safest everyday choice.

It repeats wskazówka for clarity.

The full sentence is:

• Kiedy duża wskazówka jest na dwunastej, mała wskazówka pokazuje godzinę.

You could sometimes omit the noun if the context is very clear, but repeating it makes the explanation easier to understand, especially in teaching material.

Compare:

• duża wskazówka ... mała wskazówka ... = very clear
• duża wskazówka ... mała ... = possible in context, but less explicit

Not completely. Polish word order is more flexible than English word order, because case endings show grammatical roles.

The given sentence is natural and clear:

• Kiedy duża wskazówka jest na dwunastej, mała wskazówka pokazuje godzinę.

But other orders are possible for emphasis, for example:

• Mała wskazówka pokazuje godzinę, kiedy duża wskazówka jest na dwunastej.

That said, not every rearrangement sounds equally natural. The original order is a good neutral model for learners.

Yes, this is understandable and natural, especially in explanations.

Still, in everyday speech, people may also use the more specific terms:

• wskazówka godzinowa = hour hand
• wskazówka minutowa = minute hand

So instead of:

• duża wskazówka
• mała wskazówka

you might also hear:

• duża wskazówka / mała wskazówka in simple teaching language
• wskazówka minutowa / wskazówka godzinowa in more precise language

Both approaches are useful to know.

Yes:

• Kiedy = when
• duża = big
• wskazówka = hand / pointer
• jest = is
• na = on / at
• dwunastej = twelve / the twelfth / the twelve position, in the locative form
• mała = small
• wskazówka = hand
• pokazuje = shows
• godzinę = the hour, in the accusative form

So the structure is:

• time clause: Kiedy duża wskazówka jest na dwunastej
• main clause: mała wskazówka pokazuje godzinę