Kad sam izašla na sunce, trepnula sam i stavila sunčane naočale.

Because the speaker is female.

In Croatian, the past participle agrees with the subject’s gender and number:

• izašao sam = I went out / I came out (male speaker)
• izašla sam = I went out / I came out (female speaker)

The same applies later in the sentence:

• trepnuo sam / stavio sam = male speaker
• trepnula sam / stavila sam = female speaker

So this whole sentence is being said by a woman.

This is how the Croatian past tense is normally formed.

The past tense usually uses:

• the past participle of the main verb
• plus the auxiliary biti in the present tense

So:

• sam izašla = I went out
• sam trepnula = I blinked
• sam stavila = I put on

In standard word order, the auxiliary often comes in the second position in its clause:

• Kad sam izašla na sunce...
• ...trepnula sam i stavila sunčane naočale.

In the second clause, sam is not repeated before stavila because one auxiliary can cover both coordinated verbs:

• trepnula sam i stavila...
  = I blinked and put on...

You could think of it as:

• I blinked and [I] put on sunglasses.

Kad means when.

Here it introduces a time clause:

• Kad sam izašla na sunce = When I went out into the sun

Croatian also has kada, which means the same thing. In everyday speech, kad is very common and slightly shorter/more conversational.

So:

• Kad sam izašla na sunce...
• Kada sam izašla na sunce...

Both are correct.

Here na sunce means something like:

• out into the sun
• into the sunlight

This is a very natural Croatian expression.

Why na? Because Croatian often uses na with the accusative to express movement onto or into an exposed place/state, and sunce in this expression refers more to being in the sun / sunlight than to the literal astronomical object.

Compare:

• izaći na sunce = to go out into the sun
• sjediti na suncu = to sit in the sun

Notice the difference:

• na sunce = movement toward it / into it
• na suncu = location in it

So in your sentence, na sunce is used because the speaker is moving out into the sun.

Because na sunce here uses the accusative case, due to movement.

With na, Croatian often distinguishes:

• accusative = movement toward a place
• locative = being at/in/on a place

So:

• izašla na sunce = went out into the sun → movement → accusative
• bila na suncu = was in the sun → location → locative

The noun sunce has the same form in nominative and accusative singular, so it stays sunce.

Trepnuti means to blink.

So:

• trepnula sam = I blinked

It usually refers to a quick closing and opening of the eyes, often because of bright light, surprise, or irritation.

In this sentence, it fits very naturally: the speaker went out into bright sunlight, blinked, and then put on sunglasses.

Yes. Staviti basically means to put / place, but in many contexts it is also used for putting something on.

So:

• staviti naočale = to put on glasses
• staviti kaput = to put on a coat
  though for clothing Croatian can also use other verbs depending on the item

In your sentence:

• stavila sunčane naočale = put on sunglasses

This is very natural Croatian.

Because sunčane naočale is the accusative plural, which is needed as the direct object of stavila.

The speaker put on what?
→ sunčane naočale

Naočale is a plural noun meaning glasses. It behaves like a plural item in Croatian, just as glasses does in English.

The adjective sunčane agrees with naočale in gender, number, and case.

So:

• sunčane naočale = sunglasses / sun glasses
• here they are in the form required by the verb stavila

In normal usage, yes, it is treated as a plural noun.

Just like English glasses, Croatian naočale refers to an object with two lenses and is normally used in the plural:

• Imam naočale. = I have glasses.
• Stavila sam naočale. = I put on my glasses.

So even when referring to one pair, Croatian still uses the plural form.

Because Croatian often omits subject pronouns when they are clear from the verb form.

In English, you must say:

• I went out
• I blinked
• I put on

In Croatian, the verb forms already show the person, and in the past tense the participle also shows gender:

• izašla sam
• trepnula sam
• stavila

So ja is not necessary.

You could say Ja sam izašla... if you want emphasis, but without emphasis Croatian normally leaves it out.

A natural breakdown is:

• Kad sam izašla na sunce,
  When I went out into the sun,
• trepnula sam i stavila sunčane naočale.
  I blinked and put on sunglasses.

A few useful word-order points:

1. Kad introduces the subordinate clause.
2. The auxiliary sam tends to appear in second position in the clause.
3. Croatian word order is fairly flexible, but this version sounds very natural.

So this sentence is a good example of standard, idiomatic Croatian word order.

They are perfective:

• izaći = to go out / come out
• trepnuti = to blink
• staviti = to put, put on

That matters because the sentence describes completed actions in sequence:

1. she went out into the sun
2. she blinked
3. she put on sunglasses

Perfective verbs are very common when narrating single, completed events like this.

If Croatian wanted to emphasize an ongoing or repeated action, it would more likely use imperfective verbs instead.

Yes. Only the gender-marked verb forms would change.

A male speaker would say:

Kad sam izašao na sunce, trepnuo sam i stavio sunčane naočale.

Changes:

• izašla → izašao
• trepnula → trepnuo
• stavila → stavio

Everything else stays the same.

It is the standard and most common way.

• sunčane naočale = sunglasses

People may sometimes shorten things in casual speech if the context is clear, but for learners, sunčane naočale is the safe, normal expression to use.