Razočaran sam, jer je moj odgovor bio pogrešan.

Razočaran is the masculine singular form.

It agrees with the person who is speaking:

• Razočaran sam = said by a man
• Razočarana sam = said by a woman

So this adjective changes for gender and number, just like many Croatian adjectives do.

Because sam is an unstressed auxiliary form, and Croatian clitics usually go in the second position of the clause.

So:

• Razočaran sam. = natural
• Ja sam razočaran. = also natural, with ja added
• Sam razočaran = not standard word order

In other words, sam likes to come right after the first stressed element.

It is simply omitted.

Croatian often leaves out subject pronouns when they are already clear from the verb:

• sam already tells you the subject is I
• so ja is unnecessary unless you want emphasis

Compare:

• Razočaran sam. = normal
• Ja sam razočaran. = I am disappointed, with extra emphasis

In this sentence, it behaves like an adjective in a predicate: I am disappointed.

Historically, forms like this come from participles, but for a learner it is most useful to think of razočaran / razočarana as an adjective meaning disappointed, which agrees in gender and number.

Because jer introduces a subordinate clause meaning because, and in standard Croatian writing that clause is separated by a comma.

So:

• Razočaran sam, jer je moj odgovor bio pogrešan.

In speech, you may hear little or no pause, but in writing the comma is standard.

Because je is another clitic, and clitics in Croatian usually come very early in the clause.

After jer, it is standard to place the clitic immediately after it:

• jer je moj odgovor bio pogrešan

This can feel unusual to English speakers, because English would usually keep the subject before the verb. Croatian word order is more flexible, but clitics still have special placement rules.

Because there are two different clauses:

1. Razočaran sam
   Here sam means am.

2. jer je moj odgovor bio pogrešan
   Here je bio is part of the past tense and means was.

So the sentence combines:

• a present-state clause: I am disappointed
• a past-time clause: because my answer was wrong

Bio is the past participle of biti (to be), and together with je it forms the Croatian past tense.

So:

• je bio = was

It also agrees with the subject:

• odgovor is masculine singular
• therefore the form is bio

Compare:

• Moj odgovor je bio pogrešan. = masculine noun
• Moja reakcija je bila pogrešna. = feminine noun

Because moj odgovor is the subject of the second clause, so it is in the nominative.

• moj = my
• odgovor = answer

Since odgovor is masculine singular nominative, moj is also masculine singular nominative.

Because pogrešan is an adjective describing odgovor, and it must agree with it.

Since odgovor is:

• masculine
• singular
• nominative

the adjective must also be:

• pogrešan

Compare:

• odgovor je pogrešan = masculine singular
• reakcija je pogrešna = feminine singular
• rješenja su pogrešna = neuter plural

Yes. In many situations, jer and zato što can both mean because.

So you could also say:

• Razočaran sam, zato što je moj odgovor bio pogrešan.

A few notes:

• jer is very common and straightforward
• zato što is also common, sometimes a little fuller or heavier in style

For this sentence, both are natural.

Sometimes, yes, but the nuance changes a bit.

• pogrešan = wrong, incorrect
• netočan = inaccurate, not correct factually
• kriv = guilty / at fault / wrong in some contexts

For an answer, pogrešan is very natural:

• moj odgovor je bio pogrešan

Netočan is also possible, especially in a factual or academic context:

• moj odgovor je bio netočan

Kriv is less natural here than pogrešan.

The most useful ones here are:

• č in razočaran: like ch in chocolate, but harder
• ž in pogrešan? Actually there is no ž in pogrešan, but there is š
• š in pogrešan: like sh in shoe
• j in jer and moj: like English y in yes

A rough pronunciation guide:

• Razočaran sam ≈ rah-zoh-CHAH-rahn sahm
• jer je moj odgovor bio pogrešan ≈ yehr yeh moy OHD-goh-vohr BEE-oh poh-GREH-shahn

Not perfect English-style transcription, but good enough to get started.