Spojrzałam na zegar i zauważyłam, że duża wskazówka zatrzymała się na dwunastej.

Because the speaker is female.

In Polish, past tense verbs agree with the gender of the person speaking or being described.

• spojrzałam = I looked (said by a woman)
• zauważyłam = I noticed (said by a woman)

A man would say:

• spojrzałem
• zauważyłem

So this sentence tells you not only what happened, but also that the speaker is female.

These two verbs are related, but they are not used in the same way.

• spojrzeć / spojrzałam = to look, to glance, to take a look
  This suggests a single, completed action.
• patrzeć / patrzyłam = to be looking, to watch, to look at something for some time
  This suggests a continuing action.

So:

• Spojrzałam na zegar = I glanced at the clock
• Patrzyłam na zegar = I was looking at the clock

In this sentence, the speaker means a quick look, so spojrzałam is the natural choice.

Because after spojrzeć na or patrzeć na, Polish uses na + accusative.

Here:

• zegar = nominative
• na zegar = accusative

So:

• spojrzeć na coś = to look at something
• patrzeć na coś = to look at something

Compare:

• Spojrzałam na zegar. = I looked at the clock.
• Zegar stoi na stole. = The clock is standing on the table.

In the second example, na means physical location and works differently. In your sentence, na belongs to the verb phrase spojrzeć na.

They can both sometimes be translated as I saw, but they are not quite the same.

• zobaczyłam = I saw
• zauważyłam = I noticed

Zauważyłam suggests that the speaker became aware of something, often something meaningful or unexpected.

So in this sentence:

• zauważyłam, że... = I noticed that...

This sounds more natural than zobaczyłam, że..., because the speaker is not just visually seeing the clock hand, but also realizing something about it.

Że means that and introduces a subordinate clause.

Structure:

• zauważyłam, że... = I noticed that...

So the sentence is built like this:

• Spojrzałam na zegar = I looked at the clock
• i zauważyłam = and I noticed
• że duża wskazówka zatrzymała się na dwunastej = that the big hand had stopped at twelve

In English, that is sometimes optional:

• I noticed the big hand had stopped...
• I noticed that the big hand had stopped...

In Polish, że is very commonly used here.

Wskazówka is the normal Polish word for a hand of a clock/watch.

So:

• duża wskazówka = the big hand
• more specifically, usually the minute hand

Other clock-related words:

• mała wskazówka = hour hand
• sekundnik or wskazówka sekundowa = second hand

Polish does not use a word equivalent to English hand in this context. You use wskazówka.

Also, wskazówka is feminine, which is why you get:

• duża wskazówka
• zatrzymała się

Because zatrzymać się means to stop in the sense of come to a halt.

• zatrzymać coś = to stop something
• zatrzymać się = to stop oneself / to stop

Examples:

• Zatrzymałam samochód. = I stopped the car.
• Samochód zatrzymał się. = The car stopped.

So here:

• wskazówka zatrzymała się = the hand stopped

The się is essential because the hand is not stopping something else; it is itself coming to a stop.

This is one of the trickiest parts of the sentence.

Here na dwunastej means at the twelve mark / at twelve on the clock face.

The structure is:

• zatrzymać się na + locative

So:

• zatrzymała się na dwunastej = it stopped at twelve

This is not the same as o dwunastej, which means at twelve o’clock as a time.

Compare:

• Pociąg odjechał o dwunastej. = The train left at twelve o’clock.
• Wskazówka zatrzymała się na dwunastej. = The hand stopped at the twelve mark.

Why feminine? Because there is an omitted noun understood from context, something like:

• na dwunastej godzinie
• or simply na dwunastej as a clock position

In everyday Polish, na dwunastej is completely natural in this kind of context.

Dwunastej is locative singular, feminine.

It appears because of the expression:

• na + locative after zatrzymać się na

Here the idea is to stop on/at a certain point.

So:

• dwunasta = nominative feminine
• na dwunastej = locative feminine

This agrees with an implied feminine noun, such as godzina or a clock-position idea understood from context.

Because in Polish, subordinate clauses introduced by words like że, bo, gdy, który, etc. are normally separated by a comma.

So:

• zauważyłam, że...

This comma is required in standard Polish.

English punctuation is often looser here, but Polish is stricter.

Not exactly.

• zegar = clock
• zegarek = watch or small clock, usually something personal like a wristwatch

So if you look at a wall clock, table clock, or alarm clock, zegar is natural.

Your sentence says:

• Spojrzałam na zegar = I looked at the clock

If it were a wristwatch, you would more likely say:

• Spojrzałam na zegarek

Usually yes, in ordinary everyday language.

On a standard analog clock:

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

But technically, duża just means big/large, so the phrase is descriptive, not a fixed technical term in the strictest sense. In normal speech, though, Polish speakers will understand it as the minute hand.

Grammatically yes, but it would sound less natural.

• duży / duża is the normal everyday word for big/large
• wielki / wielka often sounds stronger, more emphatic, or more like great/huge

So:

• duża wskazówka = natural
• wielka wskazówka = possible, but a bit odd unless you want special emphasis

In this context, duża is definitely the better choice.

Not if you want the same meaning.

• zatrzymała się na dwunastej = it stopped at the twelve mark on the clock
• zatrzymała się o dwunastej = it stopped at twelve o’clock

Those are different ideas.

In your sentence, the focus is on the position of the hand, not the actual time when the stopping happened. That is why na dwunastej is used.

It has two main actions, followed by a subordinate clause:

1. Spojrzałam na zegar
   I looked at the clock

2. i zauważyłam
   and I noticed

3. że duża wskazówka zatrzymała się na dwunastej
   that the big hand had stopped at twelve

So the overall pattern is:

• I looked at X and noticed that Y had happened.

This is a very common and useful Polish structure:

• Zobaczyłam, że... = I saw that...
• Usłyszałam, że... = I heard that...
• Zauważyłam, że... = I noticed that...