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

Polish often omits subject pronouns when the verb form already makes the subject clear.

Here, spojrzałam and zauważyłam both tell us:

• 1st person singular = I
• past tense
• female speaker = the ending -łam

So Spojrzałam... zauważyłam... means a woman is speaking.
If a man were speaking, it would be Spojrzałem na zegar i zauważyłem...

Spojrzeć means to glance / to take a look. It usually refers to a single, completed action.

That fits this sentence well: the speaker looked at the clock once and then noticed something.

By contrast:

• patrzeć = to look, to watch, often ongoing
• patrzyłam na zegar would sound more like I was looking at the clock

So spojrzałam is the natural choice for I looked / I glanced in this context.

Because the verb spojrzeć normally goes with na + accusative when you look at something:

• spojrzeć na zegar
• spojrzeć na drzwi
• spojrzeć na dziecko

In this sentence, zegar is in the accusative singular. It just happens to look the same as the nominative form, because zegar is a masculine inanimate noun.

So although English just says look at the clock, Polish uses the pattern spojrzeć na + accusative.

Że means that and introduces a subordinate clause:

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

In English, that is often omitted:

• I noticed the hand had stopped...

But in Polish, że is very common and usually sounds natural here.

Also, Polish requires a comma before że:

• zauważyłam, że...

Yes. Literally, duża wskazówka means big hand or large pointer.

On a clock, it normally refers to the minute hand, contrasted with:

• mała wskazówka = the hour hand

So this is a natural everyday way to talk about the hands of a clock.

Also, wskazówka can mean other things in other contexts, such as:

• a pointer
• a hint
• a clue
• an indicator

But here it clearly means the hand of a clock.

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

Compare:

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

In this sentence, the hand itself stopped, so Polish uses zatrzymała się.

Also note:

• zatrzymała is feminine singular past tense
• it agrees with wskazówka, which is feminine

So:

• wskazówka zatrzymała się = the hand stopped

Because these mean different things:

• o dwunastej = at twelve o’clock as a time
• na dwunastej = on twelve as a position on the clock face

In this sentence, the hand is not being described as stopping at noon. It is being described as stopping at the twelve mark on the dial.

So:

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

Those are different ideas.

Dwunastej is the locative singular feminine form of dwunasta.

Polish tells time using feminine ordinal numbers because the understood noun is godzina:

• pierwsza = one o’clock
• druga = two o’clock
• dwunasta = twelve o’clock

After na, in this expression, you get the locative:

• na pierwszej
• na drugiej
• na dwunastej

So dwunastej is not the basic number dwanaście. It is the ordinal-style time form used with clocks.

Yes. Polish word order is more flexible than English word order.

The original sentence is neutral and natural:

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

But other orders are possible for emphasis, for example:

• ...że duża wskazówka się zatrzymała na dwunastej
• ...że na dwunastej zatrzymała się duża wskazówka

These can sound a little more marked or emphasize different parts of the sentence.

So the original version is a very good neutral model, even though alternatives exist.

Because zegar and zegarek are related but not identical:

• zegar = clock
• zegarek = watch, or sometimes a small clock

If you look at a wall clock, desk clock, or alarm clock, zegar is natural.
If you look at your wristwatch, zegarek is more natural:

• Spojrzałam na zegarek = I looked at my watch

So in this sentence, zegar suggests a clock rather than a watch.

In Polish, a comma is normally used before że, because it introduces a subordinate clause:

• zauważyłam, że...

But there is usually no comma before i when it simply joins two coordinated parts:

• Spojrzałam na zegar i zauważyłam...

So the punctuation here follows standard Polish rules:

• no comma before i
• comma before że