Izgubljena naušnica je bila ispod kreveta.

Because it agrees with naušnica, which is a feminine singular noun.

In Croatian, adjectives change their form to match the noun’s:

• gender
• number
• case

So here:

• izgubljen = base adjective idea, lost
• izgubljena naušnica = lost earring (feminine singular nominative)

If the noun were masculine, you would get something like izgubljeni ključ or izgubljen ključ, depending on the adjective form and context.

Yes. Naušnica is a feminine noun.

A very common clue is that many Croatian nouns ending in -a are feminine:

• kuća = house
• žena = woman
• naušnica = earring

Because naušnica is feminine, other words connected to it also become feminine:

• izgubljena
• bila

This is how the past tense is normally formed in Croatian.

Croatian often makes the past tense with:

• the present tense of biti (to be), here je
• plus the past participle, here bila

So:

• je bila = was

This may look unusual to an English speaker because English uses just was, but Croatian builds it as a two-part form.

Because bila must agree with the subject, which is naušnica.

Past participles in Croatian agree in:

• gender
• number

So:

• bio = masculine singular
• bila = feminine singular
• bilo = neuter singular
• bili / bile / bila = plural forms depending on gender

Since naušnica is feminine singular, the correct form is bila.

Because ispod requires the genitive case.

The dictionary form is:

• krevet = bed

But after ispod (under / below / beneath), you need the genitive:

• ispod kreveta = under the bed

So kreveta is the genitive singular form of krevet.

Yes, in standard Croatian ispod is followed by the genitive.

Examples:

• ispod stola = under the table
• ispod mosta = under the bridge
• ispod kreveta = under the bed

This is something learners usually just have to memorize together with the preposition:

• ispod + genitive

Both can refer to something being under the bed, but they are not built the same way.

• ispod + genitive → ispod kreveta
• pod + instrumental → pod krevetom

In many situations, both are possible. Very broadly:

• ispod often feels a bit more clearly like beneath / underneath
• pod is also common for under

So this sentence could also be phrased as:

• Izgubljena naušnica je bila pod krevetom.

A learner should mainly remember:

• ispod takes genitive
• pod takes instrumental for location

Because Croatian does not have articles like English a and the.

So:

• naušnica can mean an earring or the earring
• the exact meaning depends on context

In your sentence, English naturally uses the lost earring, but Croatian simply says:

• izgubljena naušnica

Context tells you whether it is definite or indefinite.

No, Croatian word order is fairly flexible, though some orders sound more neutral than others.

Your sentence:

• Izgubljena naušnica je bila ispod kreveta.

This is a natural, neutral way to say it.

But Croatian can move parts around for emphasis, for example:

• Ispod kreveta je bila izgubljena naušnica.
  = emphasizes under the bed
• Izgubljena naušnica bila je ispod kreveta.
  = also possible, a bit more formal or stylistic

So the meaning stays similar, but the focus can shift.

Because je is a clitic, and Croatian clitics usually go near the second position in the sentence.

In:

• Izgubljena naušnica je bila ispod kreveta

the unit Izgubljena naušnica comes first, and then the clitic je appears.

This second-position tendency is very typical in Croatian. English speakers often find it strange at first, but it becomes more natural with exposure.

Yes, but it means something slightly different in structure.

• Izgubljena naušnica je bila ispod kreveta.
  = The lost earring was under the bed.
  Here izgubljena directly describes the noun: lost earring.

• Naušnica je bila izgubljena.
  = The earring was lost.
  Here izgubljena is part of the predicate, describing the state of the earring.

So both are correct, but they are not the same sentence shape.

Because it is in the nominative case, and it agrees internally:

• izgubljena = feminine singular nominative
• naušnica = feminine singular nominative

Also, the verb phrase je bila agrees with that subject in gender and number.

So the sentence structure is:

• Izgubljena naušnica = subject
• je bila = past tense of to be
• ispod kreveta = location phrase

Yes, it sounds natural and correct.

It is a straightforward sentence with:

• an adjective + noun
• a normal past tense
• a common location phrase

A native speaker would understand it immediately. It is a good example for learning:

• adjective agreement
• past tense formation
• preposition + case usage