Kad je glazba utihnula, u kući je opet bila tišina.

Because Croatian is using the past tense twice:

• je ... utihnula = has/went quiet / fell silent
• je bila = was

In Croatian, a very common past tense is formed with:

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

So:

• je utihnula = past tense of utihnuti
• je bila = past tense of biti

English often uses just one past form, but Croatian usually builds it with these two parts.

This is because je is a clitic, and Croatian clitics usually go in the second position of the clause.

Here, the clause begins with Kad (when), so je comes right after it:

• Kad je glazba utihnula

For an English speaker, this can feel unusual, because English does not have this kind of second-position clitic rule.

So the pattern is basically:

• Kad
  • je
    • rest of clause

Utihnula is the l-participle of the verb utihnuti, which means to become silent, to go quiet, or to fall silent.

In this sentence it agrees with glazba, which is a feminine singular noun, so the form is feminine singular:

• masculine: utihnuo
• feminine: utihnula
• neuter: utihnulo

So glazba je utihnula literally means something like the music became silent.

Because glazba is a feminine noun.

In Croatian past tense, the l-participle agrees with the subject in:

• gender
• number

Since glazba is feminine singular, the verb form must also be feminine singular:

• glazba je utihnula

If the subject were masculine, you would get utihnuo. If it were plural, the form would be plural.

Glazba is in the nominative singular because it is the subject of the verb utihnula.

The subject of the clause is the music, so nominative is exactly what we expect:

• glazba je utihnula = the music fell silent

Because u kući expresses location, not movement.

With u, Croatian uses:

• accusative for motion into something
• locative for being in something

So:

• u kuću = into the house
• u kući = in the house

Here the sentence says that the silence was in the house, so Croatian uses the locative:

• u kući

It is locative singular.

The base form is kuća (house). After u when we mean location, it changes to locative:

• kuća → u kući

This is a very common pattern in Croatian.

Because tišina is a feminine noun, and the past form bila agrees with it.

Here tišina is the thing that was in the house:

• u kući je opet bila tišina

Since tišina is feminine singular, Croatian uses:

• bila, not bio

Because after biti (to be), Croatian often uses the nominative for the predicate noun.

So in:

• bila tišina

tišina stays in the nominative, just like the dictionary form.

That is different from some languages where a different case might appear after to be.

Opet means again.

It tells us that the silence returned:

• first there was silence
• then music
• then, when the music stopped, there was silence again

Its position here is natural:

• u kući je opet bila tišina

Croatian word order is flexible, so opet can sometimes move, but this position is very normal and neutral.

Croatian word order is much more flexible than English word order.

The sentence puts u kući early because it helps set the scene:

• in the house, there was silence again

Also, the clitic je tends to come early in the clause, which influences the structure.

So this order sounds natural and smooth in Croatian:

• u kući je opet bila tišina

A different order may still be grammatical, but it could sound more marked or shift the emphasis.

Because the sentence begins with a subordinate clause:

• Kad je glazba utihnula = When the music fell silent

After that comes the main clause:

• u kući je opet bila tišina

Croatian normally separates that opening subordinate clause from the main clause with a comma.

It is the same basic word, but here it is being used as a subordinating conjunction, not as a direct question word.

Compare:

• Kad je došao? = When did he arrive?
• Kad je glazba utihnula, ... = When the music fell silent, ...

So in this sentence, kad introduces a time clause.

Utihnuti is perfective.

That matters because the sentence refers to a completed change:

• the music was sounding
• then it went silent

Perfective verbs in Croatian often express a single completed event or change of state. That fits perfectly here.

An English speaker can think of utihnuti as something like to become quiet / to fall silent, not simply to be quiet.

Not in standard Croatian in this sentence.

You need:

• je utihnula for the first past action
• je bila for the second past state

Croatian past tense normally requires the auxiliary je together with the participle, except in a few special styles or very informal omissions. In normal standard language, both are expected here.