Čepići za uši smanjuju buku.

Čepići is the nominative plural form. The singular is čepić.

This noun means a small plug / stopper, and in this context it means earplug. It comes from čep (plug, stopper) plus the diminutive ending -ić, so čepić literally suggests a little plug.

Because the sentence talks about earplugs in general, Croatian uses the plural čepići.

• singular: čepić
• plural: čepići

Yes, za uši literally means for ears.

This is a very natural Croatian way to say earplugs: literally plugs for ears.

• za = for
• uši = ears

So čepići za uši is a noun phrase meaning earplugs.

This kind of structure is common in Croatian:

• četkica za zube = toothbrush (literally brush for teeth)
• krema za ruke = hand cream (literally cream for hands)

Here uši is in the accusative plural.

The preposition za often takes the accusative when it means for in the sense of intended for / meant for.

The noun is irregular:

• singular: uho = ear
• plural: uši = ears

In this phrase, the accusative plural looks the same as the nominative plural: uši.

So:

• za uši = for ears
• case of uši here: accusative plural

Because the subject čepići za uši is plural.

Croatian verbs agree with the subject in number:

• čepić ... smanjuje = the earplug reduces
• čepići ... smanjuju = the earplugs reduce

So smanjuju is the 3rd person plural present tense form.

Buku is the accusative singular of buka (noise).

It is in the accusative because it is the direct object of the verb smanjuju (reduce). The earplugs are reducing what? → noise.

For many feminine nouns ending in -a, the pattern is:

• nominative singular: buka
• accusative singular: buku

So the -a changes to -u in the accusative singular.

The form smanjuju comes from the imperfective verb smanjivati, meaning to reduce or to decrease.

That is why the sentence works well as a general statement:

• Čepići za uši smanjuju buku.
• Earplugs reduce noise.

The imperfective verb is natural here because the sentence describes a general function or usual effect, not one single completed action.

These are important Croatian sounds:

• č in Čepići: a hard ch sound, like ch in chocolate, but usually a bit firmer
• ć in čepići: also a ch-like sound, but softer than č
• š in uši: like sh in shoe

A rough English-friendly pronunciation might be:

• Čepići ≈ CHEH-pee-chee
• uši ≈ OO-shee
• smanjuju ≈ SMAHN-yoo-yoo
• buku ≈ BOO-koo

The difference between č and ć can be hard for English speakers, and in some casual speech they may sound quite close, but Croatian spelling keeps them distinct.

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

So Croatian simply says:

• Čepići za uši smanjuju buku.

Depending on context, this could correspond to English as:

• Earplugs reduce noise.
• The earplugs reduce noise.
• Earplugs reduce the noise.

English must choose an article, but Croatian usually does not.

No, Croatian word order is more flexible than English word order.

The neutral, most straightforward order here is:

• Čepići za uši smanjuju buku.

This is basically subject + verb + object.

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

• Buku smanjuju čepići za uši. = It is earplugs that reduce the noise / Earplugs reduce the noise.

The basic meaning stays the same because the case endings show what each word is doing.

Yes, it is a very normal and standard way to say earplugs.

Croatian often uses descriptive phrases like this instead of a single compact noun. So čepići za uši is completely natural and common.

Learners should get used to these multi-word noun phrases, because they are very common in everyday Croatian.