Iznenadio sam se kad sam vidio koliko je more toplo.

Iznenadio sam se is the Croatian perfect tense of the verb iznenaditi se, which means to get surprised / to be surprised.

The se is part of the verb. It is not usually translated as myself here. So:

• iznenaditi = to surprise someone
• iznenaditi se = to be / get surprised

So Iznenadio sam se means I was surprised or I got surprised.

Because there are two past-tense clauses, and each one needs its own auxiliary.

• Iznenadio sam se = I was surprised
• kad sam vidio... = when I saw...

In Croatian, the perfect tense is made with:

• a form of biti (sam, si, je, smo, ste, su)
• plus the past participle

So it is completely normal to have two sam in one sentence if both clauses are in the past.

Because sam is a clitic, and Croatian clitics usually go in the second position of the clause.

So:

• kad sam vidio = correct
• kad vidio sam = not natural / incorrect in standard word order

The same thing happens later in the sentence:

• koliko je more toplo

Here je is also a clitic, so it comes right after koliko.

For the same reason: je is a clitic and normally comes in second position.

So the clause is organized like this:

• koliko = first element
• je = clitic in second position
• more = subject
• toplo = adjective

That is why koliko je more toplo is the natural order.

If you were making a simple standalone sentence, you would normally say:

• More je toplo. = The sea is warm.

Here koliko means how in the sense of how warm.

So:

• koliko je more toplo = how warm the sea is/was

In other contexts, koliko can also mean:

• how much
• how many

But in this sentence it introduces an indirect exclamative idea: I saw how warm the sea was.

The infinitive is vidjeti = to see.

Vidio is the masculine singular past participle used with sam:

• sam vidio = I saw (male speaker)

Other forms include:

• vidjela sam = I saw (female speaker)
• vidjeli smo = we saw (mixed or masculine plural)
• vidjele smo = we saw (all-female group)

So vidio is just the form that agrees with a male speaker in the singular.

Yes. In the past tense, Croatian participles agree with the subject in gender and number.

So this sentence is spoken by a male speaker:

• Iznenadio sam se kad sam vidio...

A female speaker would say:

• Iznenadila sam se kad sam vidjela koliko je more toplo.

This is one of the big differences from English: Croatian past-tense forms often show gender.

More is in the nominative because it is the subject of the clause.

• more = sea
• je toplo = is warm

More is a neuter singular noun, so the adjective must agree with it:

• toplo = neuter singular
• compare:
  • grad je topao = the city is warm (masculine)
  • voda je topla = the water is warm (feminine)
  • more je toplo = the sea is warm (neuter)

Yes. Kad and kada both mean when.

So you can also say:

• Iznenadio sam se kada sam vidio koliko je more toplo.

In many situations:

• kad sounds a bit shorter and more everyday
• kada can sound a bit more formal or more explicit

But in this sentence, both are correct.

Yes, Croatian word order is fairly flexible, but the clitics still have to stay in their normal position.

For example, you can also say:

• Kad sam vidio koliko je more toplo, iznenadio sam se.

This puts more focus on the when I saw... part first.

What you usually cannot do is move clitics like sam and je freely. So even when the sentence order changes, those little words still follow the clitic-placement rules.

Because Croatian has no articles.

So more can mean:

• sea
• the sea
• sometimes even a sea, depending on context

In this sentence, English naturally uses the sea, but Croatian does not need a separate word for that.

Yes, you could, but the nuance is slightly different.

• Iznenadio sam se... = I got surprised / I was surprised
  This focuses more on the reaction happening.
• Bio sam iznenađen... = I was surprised
  This sounds a bit more like describing your state.

In many contexts, both are possible. The original sentence with iznenadio sam se feels very natural for a reaction to discovering something.