Zbog žurbe sam zaboravio ključ.

Because zbog requires the genitive case in Croatian.

The noun is:

• žurba = the basic dictionary form, nominative singular

After zbog, it changes to genitive:

• žurbe

So:

• zbog žurbe = because of hurry / because of the rush

This is a very common pattern:

• zbog kiše = because of the rain
• zbog posla = because of work
• zbog tebe = because of you

It is genitive singular.

The noun žurba is a feminine noun, and its genitive singular form is žurbe.

So the structure is:

• zbog
  • genitive

That is why you get:

• žurba → žurbe

Sam is a clitic in Croatian. Clitics are short unstressed words that usually go in the second position of the sentence or clause.

Here, the first unit is:

• Zbog žurbe

So the clitic sam comes right after that:

• Zbog žurbe sam zaboravio ključ.

This sounds natural in Croatian.

You will often see this pattern with clitics:

• Jučer sam radio.
• U školi sam ga vidio.
• Poslije ručka smo otišli.

So even though English would place am/have differently, Croatian follows its own clitic-placement rule.

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

In:

• sam zaboravio

the sam tells you it is first person singular (I), so ja is not necessary.

You could say:

• Ja sam zaboravio ključ.

But that usually adds emphasis, contrast, or clarity:

• Ja sam zaboravio ključ, ne ti. = I forgot the key, not you.

So in a neutral sentence, leaving out ja is normal.

It is the past tense (often called the perfect in Croatian grammar).

It is formed with:

• the present tense of biti (sam, si, je, smo, ste, su)
• plus the l-participle of the main verb

Here:

• sam = I am as an auxiliary
• zaboravio = past participle form of zaboraviti

Together:

• sam zaboravio = I forgot / I have forgotten

In everyday Croatian, this is the normal way to talk about past actions.

Because the participle agrees with the gender of the speaker.

• zaboravio = masculine singular
• zaboravila = feminine singular

So:

• a male speaker says Zbog žurbe sam zaboravio ključ.
• a female speaker says Zbog žurbe sam zaboravila ključ.

This is normal in Croatian past tense.

It is in the accusative singular, because it is the direct object of zaboravio.

You forgot what?

• ključ

For many masculine inanimate nouns, the accusative singular looks the same as the nominative singular.

So:

• nominative: ključ
• accusative: ključ

That is why the form does not change.

Compare:

• Vidim stol. = I see the table.
  (stol is also masculine inanimate, so accusative = nominative)

But with masculine animate nouns, accusative usually changes:

• Vidim čovjeka. = I see the man.

Because Croatian does not have articles like a or the.

So ključ can mean:

• a key
• the key

The exact meaning depends on context.

This is normal throughout Croatian:

• Imam knjigu. = I have a/the book.
• Vidim auto. = I see a/the car.

If Croatian speakers want to be more specific, they use other words, word order, or context rather than articles.

Zaboraviti is perfective.

It is used for a completed event:

• you forgot the key at a particular moment / in a particular situation

That fits this sentence well.

The imperfective partner is usually:

• zaboravljati

Very roughly:

• zaboraviti = to forget (as a completed act)
• zaboravljati = to be forgetting / to forget repeatedly / habitually

Examples:

• Opet sam zaboravio ključ. = I forgot the key again.
• Često zaboravljam ključeve. = I often forget keys.

So in this sentence, the perfective verb is the natural choice.

Yes, absolutely. That is a very natural alternative.

There is a slight difference in nuance:

• Zbog žurbe sam zaboravio ključ.
  = Because of the hurry / rush, I forgot the key.
  This focuses more on cause.

• U žurbi sam zaboravio ključ.
  = In a hurry, I forgot the key.
  This focuses more on the state or situation you were in.

Both are common and idiomatic.

Croatian word order is fairly flexible, but not completely free.

The neutral sentence is:

• Zbog žurbe sam zaboravio ključ.

You may also hear:

• Ključ sam zaboravio zbog žurbe.
• Zaboravio sam ključ zbog žurbe.

These can shift emphasis:

• putting ključ earlier can emphasize the thing forgotten
• putting zbog žurbe later can make it sound like an explanation added afterward

But the clitic sam still tends to stay in second position within its clause.

So word order can change for style, focus, and emphasis, but clitic placement remains important.

A learner may especially notice ž and lj here.

• ž is like the s in measure or vision
• lj is a single Croatian sound, similar to the lli in some pronunciations of million, though not exactly the same
• č sounds like ch in church

Very roughly:

• žurbe ≈ zhoor-beh
• ključ ≈ klyooch

That said, Croatian spelling is very regular, so once you learn the sounds, pronunciation becomes much easier than in English.

Here it is clearly singular:

• ključ = key

The plural would be:

• ključevi = keys

So:

• Zaboravio sam ključ. = I forgot the key.
• Zaboravio sam ključeve. = I forgot the keys.

A learner may not immediately recognize this because the singular accusative form looks unchanged, but it is definitely singular here.